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/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: