Add BigRock: 2×2 mineable boulder with full mining/path/save support

A new entity for multi-tile rock formations. Same duck-typed contract as
single-tile Rock so MineProvider scans both transparently via World.rocks.

Differences from Rock:
  • Occupies a 2×2 footprint anchored at origin_tile (top-left).
  • Renders a single 32×32 Sprite2D drawn from the FG_Grasslands_Spring 2×2
    cluster sprites at (22, 3) brown and (30, 3) gray.
  • Blocks pathfinding on all four footprint tiles — pawns route around it.
  • MineProvider asks `rock.approach_tile_for(pawn.tile)` for the walk
    destination, so the pawn stands beside the boulder instead of trying to
    path into the blocked footprint. Rock returns its own tile (walkable);
    BigRock picks the nearest walkable perimeter neighbour.
  • Mining takes 480 ticks (4× Rock) and drops 4 stone, one per footprint tile.

All init work happens in setup() rather than _ready(): the calling pattern is
`add_child(big); big.setup(origin)`, and _ready fires inside add_child with
origin_tile still at its zero default — anything reading origin_tile from
_ready would stamp the pathfinder at the wrong tile.

Wired through SaveSystem: factory preload, spawn-priority tier 0 (same as
Rock — static structures spawn before pawns), and a `&"big_rock"` factory.

World seed adds two demo boulders near the small-rock cluster
(65, 58) + (56, 64) so the visual contrast is on-screen from boot.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scenes/entities/big_rock.gd

@@ -0,0 +1,240 @@
+## BigRock — 2×2 boulder formation. Same mining contract as Rock but occupies
+## four tiles, drops four stone, and blocks pathfinding on its footprint so
+## pawns route around it.
+##
+## Duck-typed to Rock's public interface (tile, is_mineable, on_mine_tick,
+## approach_tile_for) so MineProvider can scan World.rocks without checking
+## the entity's concrete type.
+##
+## Position semantics (different from single Rock):
+##   • origin_tile is the TOP-LEFT of the 2×2 footprint (player-friendly anchor).
+##   • The Sprite2D is 32×32 and is centred at the intersection of the four
+##     tiles, so its visual centre sits at (origin_tile + (1, 1)) * 16 px.
+##   • For MineProvider distance, `tile` aliases to origin_tile (cheap, stable).
+##   • For the pawn's walk destination, approach_tile_for(pawn_tile) returns
+##     the closest walkable perimeter neighbour, so the pawn stands BESIDE the
+##     boulder instead of trying to path onto a blocked tile.
+
+class_name BigRock extends Node2D
+
+const TILE_SIZE_PX: int = 16
+
+## Sim ticks to mine a big rock. Roughly 4× a small rock (120 ticks) since the
+## footprint represents four tiles' worth of stone.
+const MINE_TICKS: int = 480
+## Stone Items dropped on a successful mine (one per footprint tile).
+const STONE_DROPS_ON_MINE: int = 4
+
+## Footprint dimensions. Locked at 2×2 for the first BigRock pass; if larger
+## boulders ever ship, generalise here and the perimeter math in approach_tile_for.
+const FOOTPRINT_W: int = 2
+const FOOTPRINT_H: int = 2
+
+# Preloaded scene for spawned stone items.
+const ITEM_SCENE: PackedScene = preload("res://scenes/entities/item.tscn")
+
+## ElvGames Grasslands tileset — 2×2 cluster sprites starting at these
+## top-left coords. Visually confirmed against /tmp/rocks_labeled_grid.png
+## in the 2026-05-12 visual pass.
+const _ROCK_TEX: Texture2D = preload("res://art/tiles/FG_Grasslands_Spring.png")
+const _BIG_ROCK_ATLAS_COORDS: Array[Vector2i] = [
+	Vector2i(22, 3),  # brown 2×2 boulder
+	Vector2i(30, 3),  # gray  2×2 boulder
+]
+
+
+# ── state ─────────────────────────────────────────────────────────────────────
+
+## Top-left tile of the 2×2 footprint.
+var origin_tile: Vector2i = Vector2i.ZERO
+
+## 0..MINE_TICKS. Advanced by on_mine_tick(); rock is mined when equal to MINE_TICKS.
+var mine_progress: int = 0
+
+
+# ── lifecycle ─────────────────────────────────────────────────────────────────
+
+## All init work happens inside setup() (not _ready) because callers
+## always do `add_child(node); node.setup(origin)` — _ready fires inside
+## add_child with origin_tile still at its zero default, so anything that
+## reads origin_tile from _ready would stamp the pathfinder / position at
+## the wrong tile. setup() is the single authoritative entry point.
+func _ready() -> void:
+	pass
+
+
+func _exit_tree() -> void:
+	World.unregister_rock(self)
+	_set_footprint_walkable(true)
+
+
+# ── public API ────────────────────────────────────────────────────────────────
+
+## One-shot initialiser. Builds the sprite, positions the node, marks the four
+## footprint tiles unwalkable, and registers with World.rocks. Call after
+## add_child().
+func setup(p_origin: Vector2i) -> void:
+	origin_tile = p_origin
+	mine_progress = 0
+	position = Vector2(
+		(origin_tile.x + 1) * TILE_SIZE_PX,
+		(origin_tile.y + 1) * TILE_SIZE_PX,
+	)
+	_build_sprite()
+	_set_footprint_walkable(false)
+	World.register_rock(self)
+	queue_redraw()
+	Audit.log("big_rock", "spawned 2×2 at %s" % origin_tile)
+
+
+## Alias used by MineProvider's distance calc (`rock.tile`). Returns the
+## top-left so the manhattan distance is stable; refining to the centre would
+## require fractional tiles.
+var tile: Vector2i:
+	get:
+		return origin_tile
+
+
+## True when the boulder hasn't been fully mined yet.
+func is_mineable() -> bool:
+	return mine_progress < MINE_TICKS
+
+
+## Returns the perimeter tile closest to `pawn_tile` that the pawn can stand
+## on while mining. Mirrors Rock.approach_tile_for so MineProvider can ask the
+## entity for its walk destination without knowing which kind of rock it is.
+## Falls back to the origin tile when no perimeter is walkable — the pathfinder
+## will then return an empty path and the job runner cancels cleanly.
+func approach_tile_for(pawn_tile: Vector2i) -> Vector2i:
+	var perimeter := _perimeter_tiles()
+	var best: Vector2i = origin_tile
+	var best_d: int = 0x7fffffff
+	var found_any: bool = false
+	for t in perimeter:
+		if World.pathfinder == null:
+			continue
+		if not World.pathfinder.is_walkable(t):
+			continue
+		var d: int = abs(t.x - pawn_tile.x) + abs(t.y - pawn_tile.y)
+		if d < best_d:
+			best_d = d
+			best = t
+			found_any = true
+	if not found_any:
+		return origin_tile
+	return best
+
+
+## Called by the INTERACT toil in JobRunner once per sim tick while the pawn
+## works this boulder. Advances mine_progress and triggers mined() when done.
+func on_mine_tick() -> void:
+	if not is_mineable():
+		return
+	mine_progress += 1
+	queue_redraw()
+	if mine_progress >= MINE_TICKS:
+		mined()
+
+
+## Drop four stone Items (one per footprint tile) and free this node. Called
+## by on_mine_tick() automatically; can also be called for scripted removal.
+func mined() -> void:
+	for ft in footprint_tiles():
+		var item: Item = ITEM_SCENE.instantiate()
+		get_parent().add_child(item)
+		item.setup(Item.TYPE_STONE, 1, ft)
+	Audit.log("big_rock", "mined 2×2 at %s; %d stone drops" % [origin_tile, STONE_DROPS_ON_MINE])
+	queue_free()
+
+
+## The four tiles this boulder occupies (origin + the three south/east neighbours).
+func footprint_tiles() -> Array[Vector2i]:
+	return [
+		origin_tile,
+		origin_tile + Vector2i(1, 0),
+		origin_tile + Vector2i(0, 1),
+		origin_tile + Vector2i(1, 1),
+	]
+
+
+# ── save / load ───────────────────────────────────────────────────────────────
+
+func to_dict() -> Dictionary:
+	return {
+		"class_id": &"big_rock",
+		"origin_x": origin_tile.x,
+		"origin_y": origin_tile.y,
+		"mine_progress": mine_progress,
+	}
+
+
+static func from_dict(d: Dictionary) -> Dictionary:
+	return {
+		"origin_x": int(d.get("origin_x", 0)),
+		"origin_y": int(d.get("origin_y", 0)),
+		"mine_progress": int(d.get("mine_progress", 0)),
+	}
+
+
+# ── render ────────────────────────────────────────────────────────────────────
+
+func _draw() -> void:
+	# Sprite child draws the boulder body. _draw renders only the mine-progress
+	# crack overlay so the player sees mining damage.
+	if mine_progress > 0:
+		var ratio := float(mine_progress) / float(MINE_TICKS)
+		var crack_len := ratio * 12.0
+		draw_line(
+			Vector2(-4.0, -2.0),
+			Vector2(-4.0 + crack_len, 4.0),
+			Color(0.15, 0.12, 0.10, 0.85),
+			1.5
+		)
+
+
+# ── internal ──────────────────────────────────────────────────────────────────
+
+## Build a single 32×32 Sprite2D from a 2×2 region of FG_Grasslands_Spring.
+## Variant chosen deterministically from origin_tile so the same boulder renders
+## the same colour across boots and load/save.
+func _build_sprite() -> void:
+	var sprite := Sprite2D.new()
+	sprite.name = "Sprite"
+	sprite.texture = _ROCK_TEX
+	sprite.region_enabled = true
+	var coord_idx: int = (origin_tile.x * 31 + origin_tile.y * 17) % _BIG_ROCK_ATLAS_COORDS.size()
+	var coord: Vector2i = _BIG_ROCK_ATLAS_COORDS[coord_idx]
+	sprite.region_rect = Rect2(
+		coord.x * TILE_SIZE_PX,
+		coord.y * TILE_SIZE_PX,
+		TILE_SIZE_PX * FOOTPRINT_W,
+		TILE_SIZE_PX * FOOTPRINT_H,
+	)
+	sprite.centered = true
+	sprite.offset = Vector2.ZERO
+	add_child(sprite)
+
+
+## Mark or unmark the four footprint tiles in the pathfinder. Called from
+## _ready / _exit_tree so the boulder appears as an obstacle while it exists.
+func _set_footprint_walkable(walkable: bool) -> void:
+	if World.pathfinder == null:
+		return
+	for ft in footprint_tiles():
+		World.pathfinder.set_cell_walkable(ft, walkable)
+
+
+## The eight perimeter tiles around the 2×2 footprint (cardinal + diagonals
+## of the bounding rect). Used by approach_tile_for to find a stand-and-mine
+## tile near the pawn.
+func _perimeter_tiles() -> Array[Vector2i]:
+	var out: Array[Vector2i] = []
+	# Top + bottom rows.
+	for dx in range(-1, FOOTPRINT_W + 1):
+		out.append(origin_tile + Vector2i(dx, -1))
+		out.append(origin_tile + Vector2i(dx, FOOTPRINT_H))
+	# Left + right columns (skip the corners — already in the top/bottom rows).
+	for dy in range(0, FOOTPRINT_H):
+		out.append(origin_tile + Vector2i(-1, dy))
+		out.append(origin_tile + Vector2i(FOOTPRINT_W, dy))
+	return out

scenes/entities/big_rock.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3 uid="uid://big_rock_entity"]
+
+[ext_resource type="Script" path="res://scenes/entities/big_rock.gd" id="1_big_rock"]
+
+[node name="BigRock" type="Node2D"]
+script = ExtResource("1_big_rock")

scenes/entities/rock.gd

@@ -91,6 +91,15 @@ func is_mineable() -> bool:
 	return mine_progress < MINE_TICKS
 
 
+## Walk destination for a pawn approaching this rock. Small rocks are walkable
+## (Phase 4 simplification), so the pawn stands on the rock tile while mining.
+## BigRock overrides this to return a perimeter neighbour because its footprint
+## is blocked. MineProvider always asks the entity for this rather than reading
+## `.tile` directly, so single Rock and BigRock plug into the same walk toil.
+func approach_tile_for(_pawn_tile: Vector2i) -> Vector2i:
+	return tile
+
+
 ## Called by the INTERACT toil in JobRunner once per sim tick while the pawn
 ## works this rock. Advances mine_progress and triggers mined() when complete.
 func on_mine_tick() -> void: