fix(client): address PR #30 review — 9 items from Hoshe and Tyre

- Add SimBridge._last_snapshot = null to P3 before_test() (contamination risk)
- Clarify mono.is_visible is custom property, not CanvasItem builtin
- Camera smoothing test uses approximate equality instead of exact
- Propagate inner _encode_message errors in array/dict encoding
- Add explanatory comment on int_64 encoder branch
- Document unfalsifiable gauntlet guard as intentional future-proof
- Remove loose D-053 citation from LERP_SPEED pin
- Assert FogOverlay is sibling of FogGroup under World
- Add CANVAS_MODAL=30 (ModalLayer) assertion in P3-Z04

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

client/addons/messagepack/messagepack.gd

@@ -88,6 +88,7 @@ static func _encode_message(buffer: StreamPeerBuffer, value):
 				buffer.put_u8(types["uint_32"])
 				buffer.put_u32(value)
 			elif value < 0:
+				# Negative beyond int_32 range — encode as int_64 (0xd3)
 				buffer.put_u8(types["int_64"])
 				buffer.put_64(value)
 			else:
@@ -133,7 +134,9 @@ static func _encode_message(buffer: StreamPeerBuffer, value):
 				return ERR_INVALID_DATA
 			
 			for obj in value:
-				_encode_message(buffer, obj)
+				var inner_err = _encode_message(buffer, obj)
+				if inner_err:
+					return inner_err
 
 		TYPE_DICTIONARY:
 			var size = value.size()
@@ -150,8 +153,12 @@ static func _encode_message(buffer: StreamPeerBuffer, value):
 				return ERR_INVALID_DATA
 			
 			for key in value:
-				_encode_message(buffer, key)
-				_encode_message(buffer, value[key])
+				var key_err = _encode_message(buffer, key)
+				if key_err:
+					return key_err
+				var val_err = _encode_message(buffer, value[key])
+				if val_err:
+					return val_err
 		
 		TYPE_PACKED_BYTE_ARRAY:
 			var size = value.size()