feat(client): add batch fixture, fixture smoke test, decode_errors test

Tyre #1: Added Rust-generated input_batch_two.msgpack fixture for
bidirectional D-030 Layer 1 symmetry (Vec<PlayerInput>).
Tyre #2: Added all_fixtures_deserialize Rust test that reads every
.msgpack fixture and verifies it deserializes (corruption guard).
Hoshe #3: Added test_decode_snapshot_malformed_entities_counted test
verifying the decode_errors counter on D-010 boundary violations.
45 client tests, 54 server tests pass.

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

client/tests/test_protocol.gd

@@ -184,6 +184,26 @@ func test_decode_snapshot_missing_fields() -> void:
 	assert_that(result).is_null()
 
 
+func test_decode_snapshot_malformed_entities_counted() -> void:
+	# Snapshot with one valid and one malformed entity — decode_errors should count the bad one
+	var raw := {
+		"tick": 7,
+		"entities": [
+			{"entity_id": 1, "x": 5.0, "y": 10.0, "z": 0, "kind": "Npc"},
+			{"entity_id": 2, "broken": true},  # Missing required fields
+			{"x": 1.0},  # Missing entity_id, y, z, kind
+		],
+	}
+	var encoded: Variant = Messagepack.encode(raw)
+	assert_that(encoded.status).is_null()
+
+	var snapshot: Variant = Protocol.decode_snapshot(encoded.value)
+	assert_that(snapshot).is_not_null()
+	assert_that(snapshot.tick).is_equal(7)
+	assert_that(snapshot.entities.size()).is_equal(1)  # Only the valid entity
+	assert_that(snapshot.decode_errors).is_equal(2)  # Two malformed entities
+
+
 func test_decode_player_input_empty_bytes() -> void:
 	var result = Protocol.decode_player_input(PackedByteArray())
 	assert_that(result).is_null()
@@ -251,6 +271,21 @@ func test_encode_player_inputs_empty() -> void:
 	assert_that(raw.value.size()).is_equal(0)
 
 
+# -- Batch input fixture (D-030 Layer 1 bidirectional symmetry) ----------------
+
+func test_decode_batch_input_fixture() -> void:
+	# Rust-generated Vec<PlayerInput> fixture — verifies bidirectional Layer 1 compatibility
+	var bytes = _load_fixture("input_batch_two")
+	var raw: Variant = Messagepack.decode(bytes)
+	assert_that(raw.status).is_null()
+	assert_that(raw.value is Array).is_true()
+	assert_that(raw.value.size()).is_equal(2)
+	assert_that(raw.value[0]["tick"]).is_equal(0)
+	assert_that(raw.value[0]["action"]).is_equal("MoveNorth")
+	assert_that(raw.value[1]["tick"]).is_equal(0)
+	assert_that(raw.value[1]["action"]).is_equal("Interact")
+
+
 # -- Diagonal movement fixtures (D-030 Layer 1 cross-language) -----------------
 
 func test_decode_diagonal_fixtures() -> void: