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),

server/tests/serialization.rs

@@ -1,6 +1,7 @@
 //! IPC serialization round-trip tests (D-030 Layer 1: fixture-based).
 
 use settled_reach_server::bridge::types::*;
+use std::fs;
 
 #[test]
 fn observer_snapshot_roundtrip() {
@@ -82,6 +83,46 @@ fn all_player_action_variants_roundtrip() {
     }
 }
 
+/// All .msgpack fixtures must deserialize without error (guards against corruption in git).
+/// Snapshot fixtures deserialize as ObserverSnapshot, input_* as PlayerInput,
+/// input_batch_* as Vec<PlayerInput>.
+#[test]
+fn all_fixtures_deserialize() {
+    let fixture_dir = std::path::Path::new("../client/tests/fixtures/msgpack");
+    assert!(
+        fixture_dir.exists(),
+        "Fixture directory not found: {}",
+        fixture_dir.display()
+    );
+
+    let mut count = 0;
+    for entry in fs::read_dir(fixture_dir).expect("read fixture dir") {
+        let entry = entry.expect("read dir entry");
+        let path = entry.path();
+        if path.extension().and_then(|e| e.to_str()) != Some("msgpack") {
+            continue;
+        }
+        let name = path.file_stem().unwrap().to_str().unwrap().to_string();
+        let bytes = fs::read(&path).unwrap_or_else(|_| panic!("read fixture {}", name));
+
+        if name.starts_with("snapshot") {
+            rmp_serde::from_slice::<ObserverSnapshot>(&bytes)
+                .unwrap_or_else(|e| panic!("deserialize snapshot fixture {}: {}", name, e));
+        } else if name.starts_with("input_batch") {
+            rmp_serde::from_slice::<Vec<PlayerInput>>(&bytes)
+                .unwrap_or_else(|e| panic!("deserialize batch input fixture {}: {}", name, e));
+        } else if name.starts_with("input") {
+            rmp_serde::from_slice::<PlayerInput>(&bytes)
+                .unwrap_or_else(|e| panic!("deserialize input fixture {}: {}", name, e));
+        } else {
+            panic!("unknown fixture naming convention: {}", name);
+        }
+        count += 1;
+    }
+    assert!(count > 0, "no fixtures found");
+    eprintln!("Verified {} fixtures", count);
+}
+
 /// All EntityKind variants must survive MessagePack round-trip (D-030 Layer 1)
 #[test]
 fn all_entity_kind_variants_roundtrip() {