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.

docs/NETCODE.md

@@ -124,6 +124,40 @@ would break that test, which is the point of having it.
 The menu says so out loud when you are in a dungeon -- the mechanic only reads
 as fair if the player knows the cost before clicking.
 
+## Maps are streamed, never sent
+
+A dungeon's geometry reaches a client as **chunks around that client's own
+player**, and nothing else. In particular the server does *not* send the
+generation seed, even though maps are a pure function of (kind, seed, depth) and
+sending two integers would be far cheaper: a client holding the seed can
+regenerate the entire dungeon locally, which is a complete map hack requiring no
+skill. Streaming bounds what a modified client can know to roughly where its
+player has physically been.
+
+`MAP_STREAM_RADIUS` (900u) is deliberately wider than `FOG_VIEW_RADIUS` (460u).
+The client needs real geometry it cannot see, because it predicts its own
+movement against walls and simulates bullets that die on them. The accepted
+trade is that a cheater sees somewhat further than the fog shows -- what they
+cannot get is the floor plan.
+
+Two consequences fall out of partial knowledge:
+
+- **Wall deaths are announced.** A bullet dying to its lifetime or by leaving
+  the map is derivable from the spawn event and the map's dimensions, so it
+  costs nothing. A bullet dying against a *wall* is not derivable by a client
+  that has not been streamed that wall, so the server sends an explicit
+  `BULLET_DESPAWN`. Getting the order of those two checks wrong is easy and was
+  caught by a test: out-of-bounds tiles read as `WALL` by design, so testing
+  geometry before bounds reports every bullet that merely left the map as a
+  wall kill.
+- **Unknown tiles are treated as empty**, not solid, so an unstreamed region can
+  never wrongly stop a prediction. The stream radius keeps far enough ahead of
+  the player that this never decides anything visible.
+
+Hard fog is therefore a *rendering* rule, not a secrecy mechanism. The secrecy
+lives in what the server declines to send: terrain outside the stream radius,
+and (next stage) actors outside it.
+
 ## No contact damage
 
 Nothing hurts you by touching it. Every threat is a bullet you can see and