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/core/sim_config.gd

@@ -0,0 +1,64 @@
+class_name SimConfig
+extends RefCounted
+## Tuning constants shared verbatim by the server simulation and the client
+## replica. Nothing here may differ between the two builds -- if a value needs
+## to differ, it belongs in [GameConfig], not here.
+
+# --- Time -------------------------------------------------------------------
+const TICK_RATE := 60
+const TICK_DELTA := 1.0 / 60.0
+## Server sends a snapshot every N ticks (60 / 3 = 20 Hz).
+const SNAPSHOT_INTERVAL := 3
+## How far behind the newest snapshot the client renders remote actors.
+const INTERPOLATION_DELAY_TICKS := 6
+
+# --- Arena ------------------------------------------------------------------
+const ARENA_HALF := Vector2(620.0, 340.0)
+## Bullets are culled once they leave the arena by this margin.
+const BULLET_CULL_MARGIN := 64.0
+
+# --- Player -----------------------------------------------------------------
+const PLAYER_SPEED := 240.0
+const PLAYER_RADIUS := 9.0
+const PLAYER_MAX_HP := 100
+const PLAYER_FIRE_COOLDOWN := 7          # ticks
+const PLAYER_BULLET_SPEED := 620.0
+const PLAYER_BULLET_RADIUS := 4.0
+const PLAYER_BULLET_LIFETIME := 90       # ticks
+const PLAYER_BULLET_DAMAGE := 6
+const PLAYER_IFRAMES := 36               # ticks of invulnerability after a hit
+const PLAYER_RESPAWN_DELAY := 180        # ticks
+
+# --- Anti-cheat guards ------------------------------------------------------
+## Inputs older than this (relative to the newest accepted) are discarded.
+const INPUT_MAX_AGE := 30
+## Inputs claiming to be further ahead than this of the server tick are clamped.
+const INPUT_MAX_LEAD := 12
+## Hard ceiling on inputs consumed from one peer in a single tick.
+const INPUT_MAX_PER_TICK := 4
+
+# --- Emergency escape -------------------------------------------------------
+const ESCAPE_CHANNEL_TICKS := 180        # 3 seconds
+## Taking damage while channelling cancels the escape.
+const ESCAPE_BREAK_ON_DAMAGE := true
+
+# --- Bullets ----------------------------------------------------------------
+const MAX_BULLETS := 4096
+const TEAM_PLAYER := 0
+const TEAM_ENEMY := 1
+
+# --- Bullet visual kinds (index into the renderer's atlas) ------------------
+const KIND_PLAYER_SHOT := 0
+const KIND_ORB := 1
+const KIND_NEEDLE := 2
+const KIND_HEAVY := 3
+
+# --- Instances --------------------------------------------------------------
+const LOBBY_INSTANCE_ID := 1
+const DUNGEON_PARTY_MAX := 4
+## How long a forming dungeon waits for more players before it locks.
+const DUNGEON_FORMING_TICKS := 300
+
+# --- Portal -----------------------------------------------------------------
+const PORTAL_POS := Vector2(0.0, -220.0)
+const PORTAL_RADIUS := 60.0