Initial commit: Transcience MVP

Top-down twin-stick bullet-hell, Godot 4.7, server-authoritative dedicated
server with client-side prediction. Clients send input only; the server
resolves every hit for both players and enemies (no PvP).

- SimWorld: whole simulation as plain RefCounted objects (no nodes, no
  physics server), ~0.24ms/tick at peak load -- runs headless for free and
  drives 78 tests in under a second
- BulletPool: struct-of-arrays bullet storage, replicated as spawn/despawn
  events rather than per-tick state
- Emitter framework (Ring/AimedSpread/WallGap/ArcSweep) shared by trash
  enemies and bosses -- a new boss is data in src/content/content.gd, no
  simulation changes
- The Warden of the Fold: stationary 4-phase boss built entirely on that
  format
- Lobby hub with a portal into on-demand dungeon instances; one process
  hosts the hub plus every concurrent dungeon
- Emergency escape: 3s server-owned channel, cancelled by damage
- tools/check.sh, test.sh (GUT), smoke.sh (real server + bot clients over
  ENet), bench.gd; git hooks wired to the same scripts
- docs/ARCHITECTURE.md, NETCODE.md, WORKFLOW.md, ROADMAP.md

src/net/protocol.gd

@@ -0,0 +1,24 @@
+class_name Protocol
+extends RefCounted
+## Wire constants. Bump [constant VERSION] whenever a codec layout changes; the
+## server refuses mismatched clients at handshake rather than desyncing later.
+
+const VERSION := 1
+const DEFAULT_PORT := 27015
+const MAX_CLIENTS := 32
+
+## ENet channels. Separating them stops a burst of reliable bullet events from
+## head-of-line blocking the unreliable snapshot stream.
+const CH_CONTROL := 1   ## handshake, instance transitions -- reliable
+const CH_SNAPSHOT := 2  ## world state -- unreliable, newest wins
+const CH_EVENTS := 3    ## bullet spawns/despawns, hits -- reliable ordered
+const CH_INPUT := 4     ## client -> server intent -- unreliable ordered
+## Must be identical on both ends, and larger than the highest channel above.
+const CHANNEL_COUNT := 8
+
+enum InstanceKind { LOBBY, DUNGEON }
+
+## Player flags packed into the snapshot's per-player byte.
+const F_ALIVE := 1
+const F_INVULN := 2
+const F_ESCAPING := 4