feat(simulation): add world_seed IPC and EntanglementConfig (#175, #178)

Implements the StartupMessage protocol: client generates world_seed in
SessionManager.new_game(), sends it after handshake, server uses it to
seed SimRng and sample EntanglementConfig.

EntanglementConfig samples flat ∈ [25,35]%, intrigue ∈ [15,25]%, mundane
as remainder (D-029). Same seed produces identical config (D-010
determinism). Different seeds produce distinct configs in ≥90% of pairs.

Protocol flow: HandshakeMessage (server→client) → StartupMessage with
world_seed (client→server) → SimRng initialization → tick loop.

10 Rust tests (determinism, variation, bounds, sum invariant).
9 GDScript test stubs + 2 encode tests for client-side pipeline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/autoloads/game_state.gd

@@ -66,6 +66,12 @@ var gauntlet_mode: bool = false  # true when snapshot includes gauntlet_mode fla
 # the server's "insert_active" snapshot field, disabling all z-layer-6 UI.
 var insert_active: bool = true
 
+# #175: World seed for deterministic simulation (D-010, D-029).
+# Set by SessionManager.new_game(), sent to server via StartupMessage in SimBridge.
+# Same seed → same EntanglementConfig → same NPC population across playthroughs.
+# Persists for the session lifetime; not overwritten by apply_snapshot().
+var world_seed: int = 0
+
 # #507: RNG seed for replay determinism — populated from snapshot "rng_seed" field.
 # Null in v0.1 (server does not yet send this field; protocol change required).
 var rng_seed: Variant = null