fix(client): address PR #4 round 2 review feedback

- decode_snapshot() reports dropped entities via push_error and returns
  decode_errors count so callers can detect partial data (D-010
  information boundary compliance)
- receive_bytes() warns when overwriting unconsumed snapshot, documents
  latest-wins semantics
- Rename misleading test to test_encode_produces_nonempty_bytes
- Fix tick rate comment: 10 ticks/game-minute per D-031

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

client/tests/test_protocol.gd

@@ -164,7 +164,6 @@ func test_decode_player_input_empty_bytes() -> void:
 	assert_that(result).is_null()
 
 
-func test_encode_returns_empty_on_failure() -> void:
-	# Verify that a valid encode produces non-empty bytes
+func test_encode_produces_nonempty_bytes() -> void:
 	var bytes = Protocol.encode_player_input(1, "MoveNorth")
 	assert_that(bytes.size()).is_greater(0)