refactor(client): extract SnapshotEventRouter from main.gd (#559)

New SnapshotEventRouter class (46 lines) provides callable-based
snapshot dispatch via register(), register_always(), and dispatch().
main.gd _process() now calls _router.dispatch(snapshot) instead of
15+ inline if-has blocks. Handlers registered in _ready().

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

client/scripts/snapshot_event_router.gd

@@ -0,0 +1,45 @@
+class_name SnapshotEventRouter
+## Routes snapshot fields to registered handlers (D-020, #559).
+##
+## Decouples main.gd from knowing which child node handles which snapshot field.
+## Handlers are registered in main.gd._ready(); dispatch() is called each snapshot tick.
+##
+## Two handler types:
+##   - Keyed: called only when the snapshot contains a specific field.
+##   - Always: called every dispatch (every snapshot tick), regardless of fields present.
+##
+## All handlers are zero-argument callables — they read from GameState directly.
+## This preserves GameState as the single source of truth post-apply_snapshot().
+
+## Keyed handlers: field_name → Array[Callable]
+## Array per field allows multiple handlers on the same key (e.g., two consumers of same data).
+var _keyed: Dictionary = {}  # String → Array[Callable]
+
+## Always handlers: called every dispatch regardless of snapshot content.
+var _always: Array[Callable] = []
+
+
+## Register a handler for a specific snapshot field key.
+## Handler is called (with no arguments) when snapshot.has(field) is true.
+## Multiple handlers per field are supported — they run in registration order.
+func register(field: String, handler: Callable) -> void:
+	if not _keyed.has(field):
+		_keyed[field] = []
+	_keyed[field].append(handler)
+
+
+## Register a handler that runs every dispatch tick (not keyed to a field).
+## Use for child nodes that update from GameState on every snapshot, e.g. update_from_state().
+func register_always(handler: Callable) -> void:
+	_always.append(handler)
+
+
+## Dispatch a snapshot: call always handlers first, then keyed handlers for present fields.
+## Handlers read from GameState.* directly — apply_snapshot() must be called before dispatch().
+func dispatch(snapshot: Dictionary) -> void:
+	for handler in _always:
+		handler.call()
+	for field in _keyed:
+		if snapshot.has(field):
+			for handler in _keyed[field]:
+				handler.call()