Stage 1: tile maps, walls, fog of war, aggro, scrolling camera

Replaces the fixed centred arena with per-world tile geometry, which is the
foundation the remaining features sit on.

- MapGrid: tile grid with three independent flags -- blocks movement, bullets,
  sight -- so a pit stops feet but not bullets or eyes, and a barricade stops
  feet and bullets but not eyes. Circle collision with per-axis sliding, and
  Bresenham line of sight shared by fog and aggro.
- MapGen: rooms and corridors generated from (seed, depth), with hand-authored
  boss arenas from Rooms stamped in first so a corridor can never carve through
  a designed fight. Reachability from spawn to boss is asserted over 40 seeds --
  "usually connected" is the failure mode that ruins one run in twenty.
- Dungeons are populated at creation, per room, instead of gating on waves. You
  explore and choose your fights; the run ends when the boss dies, not when the
  map is swept. Boss rooms have no lock, so walking out is always available.
- Aggro: enemies need range AND line of sight, so a dungeon stays quiet until
  engaged and cover actually protects.
- Hard fog, scrolling camera, and terrain rendering.

Maps are streamed per peer in chunks around that peer's player, and the seed is
deliberately NOT sent -- a client holding it could regenerate the whole dungeon,
which is a map hack for free. Stream radius (900u) is wider than view radius
(460u) because the client predicts movement against walls and simulates bullets
that die on them; the accepted cost is a cheater seeing a little further than
the fog, never the floor plan.

Partial map knowledge means wall deaths must be announced rather than derived.
A test caught the subtle half of that: out-of-bounds tiles read as WALL by
design, so checking geometry before bounds reported every bullet leaving the map
as a wall kill.

128 tests (was 103); check.sh, test.sh and smoke.sh pass. 0.26 ms/tick with 4
players and a boss, ~65x headroom.

src/sim/patterns/wall_gap_emitter.gd

@@ -15,18 +15,22 @@ extends BulletEmitter
 @export var randomize_gap: bool = false
 
 
-func _axis() -> Dictionary:
+## Direction of travel, the axis the curtain is strung along, and how far the
+## curtain reaches -- all measured from the room the pattern is filling rather
+## than from a fixed arena.
+func _axis(bounds: Rect2) -> Dictionary:
+	var half := bounds.size * 0.5
 	match direction:
-		1: return {"dir": Vector2.UP, "along": Vector2.RIGHT, "extent": SimConfig.ARENA_HALF.x, "edge": SimConfig.ARENA_HALF.y}
-		2: return {"dir": Vector2.RIGHT, "along": Vector2.DOWN, "extent": SimConfig.ARENA_HALF.y, "edge": SimConfig.ARENA_HALF.x}
-		3: return {"dir": Vector2.LEFT, "along": Vector2.DOWN, "extent": SimConfig.ARENA_HALF.y, "edge": SimConfig.ARENA_HALF.x}
-		_: return {"dir": Vector2.DOWN, "along": Vector2.RIGHT, "extent": SimConfig.ARENA_HALF.x, "edge": SimConfig.ARENA_HALF.y}
+		1: return {"dir": Vector2.UP, "along": Vector2.RIGHT, "extent": half.x, "edge": half.y}
+		2: return {"dir": Vector2.RIGHT, "along": Vector2.DOWN, "extent": half.y, "edge": half.x}
+		3: return {"dir": Vector2.LEFT, "along": Vector2.DOWN, "extent": half.y, "edge": half.x}
+		_: return {"dir": Vector2.DOWN, "along": Vector2.RIGHT, "extent": half.x, "edge": half.y}
 
 
 func fire(ctx: EmitContext) -> void:
 	if count <= 0:
 		return
-	var ax := _axis()
+	var ax := _axis(ctx.bounds)
 	var dir: Vector2 = ax["dir"]
 	var along: Vector2 = ax["along"]
 	var extent: float = ax["extent"]
@@ -38,7 +42,8 @@ func fire(ctx: EmitContext) -> void:
 	else:
 		gap = posmod(gap_index + gap_step * ctx.shot_index, maxi(count - gap_width + 1, 1))
 
-	var start := -dir * (edge + 8.0)
+	var centre := ctx.bounds.get_center()
+	var start := centre - dir * (edge + 8.0)
 	var angle := dir.angle()
 	for i in count:
 		if i >= gap and i < gap + gap_width: