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.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")