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>

server/tests/gen_fixtures.rs

@@ -115,6 +115,22 @@ fn generate_msgpack_fixtures() {
         &rmp_serde::to_vec_named(&snapshot_multi).unwrap(),
     );
 
+    // Batch input: Vec<PlayerInput> with two actions (D-030 Layer 1 bidirectional symmetry)
+    let input_batch = vec![
+        PlayerInput {
+            tick: 0,
+            action: PlayerAction::MoveNorth,
+        },
+        PlayerInput {
+            tick: 0,
+            action: PlayerAction::Interact,
+        },
+    ];
+    write_fixture(
+        "input_batch_two",
+        &rmp_serde::to_vec_named(&input_batch).unwrap(),
+    );
+
     // Diagonal movement fixtures (clockwise: NE, SE, SW, NW)
     for (name, action) in [
         ("input_move_northeast", PlayerAction::MoveNortheast),