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/core/game_opts.gd

@@ -27,6 +27,9 @@ static var listen: bool = false
 ## Server-side dev switch: new dungeons open straight onto the boss, skipping
 ## the trash waves. For iterating on a pattern without clearing two waves first.
 static var boss_rush: bool = false
+## Dev switch: depth of newly opened dungeons, which drives map size. Depth
+## progression is a later-stage concern; this makes big maps testable now.
+static var dungeon_depth: int = 1
 static var parsed: bool = false
 
 
@@ -51,6 +54,10 @@ static func parse(argv: PackedStringArray = PackedStringArray()) -> void:
 				listen = true
 			"--boss-rush":
 				boss_rush = true
+			"--depth":
+				i += 1
+				if i < argv.size():
+					dungeon_depth = int(argv[i])
 			"--port":
 				i += 1
 				if i < argv.size():