add reachability pre-checks to plant/sleep/chop/mine

Provider audit found 6 WorkProviders missing reachability gates before
returning jobs. Without them, pawns can be offered doomed walk-jobs
(target boxed in), JobRunner cancels each tick, Decision re-offers
same job → 20Hz busy-spin starves lower-priority work.

Fixed 4 here (mechanical pattern):
- PlantProvider._find_harvest: walkable-target check (mirrors _find_sow)
- SleepProvider: walkable bed-tile check
- ChopProvider: adjacent-walkable for impassable tree
- MineProvider: adjacent-walkable for impassable rock

Cooking/Crafting reachability changes (in the same audit's
recommendation) were attempted but caused intermittent null returns
that regressed cooking rate. Reverted those — they need more careful
work that doesn't break the existing flow. Filed separately.

Future cleanup: _find_adjacent_walkable duplicated across
ConstructionProvider, ChopProvider — extract to a base/util.

MCP-verified after revert: 2 meals + 1 bread + 2 grain in cabin crate
within 3700 ticks at ULTRA. Cooking fires, hauling fires, all
production paths operational.

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

scenes/ai/chop_provider.gd

@@ -10,8 +10,9 @@ class_name ChopProvider extends WorkProvider
 ## reached.  The toil finishes automatically when is_choppable() returns
 ## false (felled) or when the node is freed.
 ##
-## Phase 4 simplification: trees do not block pathfinding, so walking directly
+## Reachability: trees are impassable, so the pawn stands at a walkable tile
-## to tree.tile is valid.  No adjacency-offset needed.
+## adjacent to the tree. A reachability pre-check on that adjacent tile prevents
+## a busy-spin when all approach tiles are blocked.
 ##
 ## Duck-typing note: Tree is referenced without class_name (class may not be
 ## registered yet when this provider loads).  We rely only on:
@@ -40,6 +41,14 @@ func find_best_for(pawn) -> Job:
 			continue
 		if Job.is_target_taken_by_other(tree, pawn):
 			continue
+		# Reachability pre-check — trees are impassable, pawn approaches from adjacent
+		# tile. Mirrors ConstructionProvider Pattern B. Skip if no walkable neighbour
+		# exists or none is reachable.
+		var approach: Vector2i = _find_adjacent_walkable(tree.tile, pawn.tile)
+		if approach == tree.tile:
+			continue  # no walkable neighbour at all
+		if pawn.tile != approach and World.pathfinder.find_path(pawn.tile, approach).is_empty():
+			continue
 		var d: int = abs(tree.tile.x - pawn.tile.x) + abs(tree.tile.y - pawn.tile.y)
 		if d < best_dist:
 			best_dist = d
@@ -51,6 +60,33 @@ func find_best_for(pawn) -> Job:
 	var j := Job.new()
 	j.label = "Chop tree at %s" % best.tile
 	j.target_node = best
-	j.toils.append(Toil.walk_to(best.tile))
+	# Walk to adjacent walkable tile — trees are impassable.
+	var stand_tile: Vector2i = _find_adjacent_walkable(best.tile, pawn.tile)
+	j.toils.append(Toil.walk_to(stand_tile))
 	j.toils.append(Toil.interact(best.get_path(), &"on_chop_tick"))
 	return j
+
+
+# ── helpers ──────────────────────────────────────────────────────────────────
+
+## Finds the 4-neighbour of `target` nearest to `prefer_near` that the pathfinder
+## currently treats as walkable. Returns `target` itself if no walkable neighbour
+## exists (caller treats that as "no approach available").
+## Mirrors ConstructionProvider._find_adjacent_walkable.
+func _find_adjacent_walkable(target: Vector2i, prefer_near: Vector2i) -> Vector2i:
+	var offsets: Array[Vector2i] = [
+		Vector2i(0, -1), Vector2i(1, 0), Vector2i(0, 1), Vector2i(-1, 0),
+	]
+	var best: Vector2i = target
+	var best_dist: int = 999999
+	for off in offsets:
+		var t: Vector2i = target + off
+		if World.pathfinder == null:
+			continue
+		if not World.pathfinder.is_walkable(t):
+			continue
+		var d: int = abs(t.x - prefer_near.x) + abs(t.y - prefer_near.y)
+		if d < best_dist:
+			best_dist = d
+			best = t
+	return best

scenes/ai/mine_provider.gd

@@ -43,6 +43,15 @@ func find_best_for(pawn) -> Job:
 			continue
 		if Job.is_target_taken_by_other(rock, pawn):
 			continue
+		# Reachability pre-check — rocks are impassable. BigRocks expose
+		# approach_tile_for(); single rocks return their own tile (walkable per
+		# Phase 4). Either way, verify the approach tile is reachable before scoring.
+		var approach: Vector2i = (
+			rock.approach_tile_for(pawn.tile) if rock.has_method("approach_tile_for") else rock.tile
+		)
+		if pawn.tile != approach and World.pathfinder != null:
+			if World.pathfinder.find_path(pawn.tile, approach).is_empty():
+				continue
 		var d: int = abs(rock.tile.x - pawn.tile.x) + abs(rock.tile.y - pawn.tile.y)
 		if d < best_dist:
 			best_dist = d

scenes/ai/plant_provider.gd

@@ -63,6 +63,10 @@ func _find_harvest(pawn) -> Job:
 			continue
 		if Job.is_target_taken_by_other(crop, pawn):
 			continue
+		# Reachability pre-check — mirrors _find_sow / HaulingProvider pattern.
+		if pawn.tile != crop.tile and World.pathfinder != null:
+			if World.pathfinder.find_path(pawn.tile, crop.tile).is_empty():
+				continue
 		var d: int = abs(crop.tile.x - pawn.tile.x) + abs(crop.tile.y - pawn.tile.y)
 		if d < best_dist:
 			best_dist = d

scenes/ai/sleep_provider.gd

@@ -58,6 +58,11 @@ func find_best_for(pawn) -> Job:
 		# nearest bed and then losing the race at claim time.
 		if Job.is_target_taken_by_other(bed, pawn):
 			continue
+		# Reachability pre-check — beds are walkable tiles, use Pattern A.
+		# Prevents a busy-spin if all beds are walled off from this pawn.
+		if pawn.tile != bed.tile and World.pathfinder != null:
+			if World.pathfinder.find_path(pawn.tile, bed.tile).is_empty():
+				continue
 		var d: int = abs(bed.tile.x - pawn.tile.x) + abs(bed.tile.y - pawn.tile.y)
 		if d < best_dist:
 			best_dist = d