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/protocol/protocol.gd

@@ -427,6 +427,18 @@ static func _decode_enum_variant(raw) -> Dictionary:
 
 # -- Encode: GDScript types → bytes to server ----------------------------------
 
+## Encode a StartupMessage to MessagePack bytes (#175).
+## Sent by the client immediately after handshake validation.
+## Server reads this to initialize SimRng with the world seed (D-010, D-029).
+static func encode_startup_message(world_seed: int) -> PackedByteArray:
+	var msg := {"world_seed": world_seed}
+	var result = Messagepack.encode(msg)
+	if result.status != null:
+		push_error("Protocol: startup message encode failed: %s" % result.status)
+		return PackedByteArray()
+	return result.value
+
+
 ## Encode a PlayerInput to MessagePack bytes.
 ## action_name: one of "MoveNorth", "MoveSouth", "MoveEast", "MoveWest",
 ##              "Interact", "UsePerceptionMode", "Pause", "Unpause"