fix: レビュー指摘事項の修正（エラー戻り値統一、タイ延長、ネストリピート、専用エラーコード）

High severity:
- compose action エラー戻り値の validation キー統一（server.py）
- ppmck/pyxel タイ & の duration 延長修正（_tie_pending フラグ導入）
- pyxel ネストリピート対応（RepeatFrame + イベントスナップショット方式）

Medium severity:
- 専用エラーコード追加: VALIDATION_MISSING_PARAMETER, VALIDATION_INVALID_MODE, VALIDATION_INVALID_ACTION
- SYSTEM_INTERNAL_ERROR キャッチ範囲拡大（compose/validate全体をtry/except）

Low severity:
- 設計書 line/column を1始まりに明記、APIレベルエラーコード追記

テスト: 129 passed, ruff: All checks passed

3 files changed, 80 insertions, 6 deletions

diff --git a/tests/test_compose_mml_tool.py b/tests/test_compose_mml_tool.py
index af1e3e2..eb1b093 100644
--- a/tests/test_compose_mml_tool.py
+++ b/tests/test_compose_mml_tool.py
@@ -85,13 +85,13 @@ def test_template_invalid_fallback(compose_mml):
 def test_compose_missing_mml(compose_mml):
     result = compose_mml(action="compose", mml="", mode="ppmck")
     assert result["success"] is False
-    assert any(e["severity"] == "error" for e in result["errors"])
+    assert any(e["severity"] == "error" for e in result["validation"]["errors"])
 
 
 def test_compose_missing_mode(compose_mml):
     result = compose_mml(action="compose", mml="A c", mode="")
     assert result["success"] is False
-    assert any(e["severity"] == "error" for e in result["errors"])
+    assert any(e["severity"] == "error" for e in result["validation"]["errors"])
 
 
 def test_validate_missing_mode(compose_mml):
@@ -103,7 +103,7 @@ def test_compose_unknown_mode(compose_mml):
     result = compose_mml(action="compose", mml="A c", mode="xyz")
     assert result["success"] is False
     assert any(
-        e["code"] == "SYNTAX_INVALID_TOKEN" for e in result["validation"]["errors"]
+        e["code"] == "VALIDATION_INVALID_MODE" for e in result["validation"]["errors"]
     )
 
 
@@ -127,7 +127,7 @@ def test_compose_with_warning(compose_mml, tmp_output_dir):
 def test_action_invalid(compose_mml):
     result = compose_mml(action="unknown")
     assert result["success"] is False
-    assert any("action" in e["message"] for e in result["errors"])
+    assert any("action" in e["message"] for e in result["validation"]["errors"])
 
 
 def test_compose_sample_rates(compose_mml, tmp_output_dir):
diff --git a/tests/test_error_codes.py b/tests/test_error_codes.py
index 19b4575..496ce84 100644
--- a/tests/test_error_codes.py
+++ b/tests/test_error_codes.py
@@ -17,14 +17,16 @@ def _find_code(result, code, key="errors"):
 
 
 def _has_error(result):
-    return any(e["severity"] == "error" for e in result["validation"]["errors"])
+    if "validation" in result:
+        return any(e["severity"] == "error" for e in result["validation"]["errors"])
+    return any(e["severity"] == "error" for e in result["errors"])
 
 
 # --- SYNTAX_INVALID_TOKEN ---
 def test_syntax_invalid_token_mode(compose_mml):
     result = compose_mml(action="compose", mml="A c", mode="xyz")
     assert result["success"] is False
-    assert _find_code(result, ErrorCode.SYNTAX_INVALID_TOKEN.value)
+    assert _find_code(result, ErrorCode.VALIDATION_INVALID_MODE.value)
 
 
 # --- SYNTAX_INVALID_NUMBER ---
@@ -201,6 +203,27 @@ def test_syntax_unterminated_header(compose_mml):
     assert _find_code(result, ErrorCode.SYNTAX_UNTERMINATED_HEADER.value)
 
 
+# --- VALIDATION_MISSING_PARAMETER ---
+def test_validation_missing_parameter(compose_mml):
+    result = compose_mml(action="compose", mml="", mode="ppmck")
+    assert result["success"] is False
+    assert _find_code(result, ErrorCode.VALIDATION_MISSING_PARAMETER.value)
+
+
+# --- VALIDATION_INVALID_MODE ---
+def test_validation_invalid_mode(compose_mml):
+    result = compose_mml(action="compose", mml="A c", mode="xyz")
+    assert result["success"] is False
+    assert _find_code(result, ErrorCode.VALIDATION_INVALID_MODE.value)
+
+
+# --- VALIDATION_INVALID_ACTION ---
+def test_validation_invalid_action(compose_mml):
+    result = compose_mml(action="unknown")
+    assert result["success"] is False
+    assert _find_code(result, ErrorCode.VALIDATION_INVALID_ACTION.value)
+
+
 # --- SYSTEM_SYNTHESIS_FAILED ---
 def test_system_synthesis_failed(compose_mml):
     with patch(
@@ -234,6 +257,20 @@ def test_system_wav_write_failed(tmp_path):
 
 
 # --- SYSTEM_INTERNAL_ERROR ---
+def test_system_internal_error(compose_mml):
+    with patch(
+        "mml_composemusic_mcp.server._parse_mml",
+        side_effect=RuntimeError("unexpected internal error"),
+    ):
+        result = compose_mml(
+            action="compose",
+            mml="A t120 l4 o4\n  c",
+            mode="ppmck",
+        )
+    assert result["success"] is False
+    assert _find_code(result, ErrorCode.SYSTEM_INTERNAL_ERROR.value)
+
+
 def test_system_internal_error_not_exposed(compose_mml):
     result = compose_mml(action="compose", mml="A t120 l4 o4\n  c", mode="ppmck")
     assert isinstance(result, dict)
diff --git a/tests/test_parser_detailed.py b/tests/test_parser_detailed.py
index 88d7095..3752562 100644
--- a/tests/test_parser_detailed.py
+++ b/tests/test_parser_detailed.py
@@ -143,6 +143,28 @@ def test_pyxel_repeat_count3():
     assert len(notes) == 6
 
 
+def test_pyxel_nested_repeat():
+    source = "0: T120 L4 O4\n[ [ C D ]2 E ]2"
+    tokens = tokenize(source, "pyxel")
+    ns, errors = parse_pyxel(source, tokens)
+    assert not any(e.severity == "error" for e in errors)
+    notes = [e for e in ns.channels["Pulse1"].events if isinstance(e, NoteEvent)]
+    # Inner: [C D]2 -> C D C D (4 notes)
+    # Outer: [ ... E ]2 -> (C D C D E) * 2 = 10 notes
+    assert len(notes) == 10
+
+
+def test_pyxel_nested_repeat_count3():
+    source = "0: T120 L4 O4\n[ [ C D ]2 E ]3"
+    tokens = tokenize(source, "pyxel")
+    ns, errors = parse_pyxel(source, tokens)
+    assert not any(e.severity == "error" for e in errors)
+    notes = [e for e in ns.channels["Pulse1"].events if isinstance(e, NoteEvent)]
+    # Inner: [C D]2 -> C D C D (4 notes)
+    # Outer: [ ... E ]3 -> (C D C D E) * 3 = 15 notes
+    assert len(notes) == 15
+
+
 # --- Tie ---
 
 
@@ -152,6 +174,21 @@ def test_ppmck_tie_note():
     ns, errors = parse_ppmck(source, tokens)
     # Tie should merge notes; check no error
     assert not any(e.severity == "error" for e in errors)
+    notes = [e for e in ns.channels["Pulse1"].events if isinstance(e, NoteEvent)]
+    # c4 & c4 should produce a single note with doubled duration (192*2=384)
+    assert len(notes) == 1
+    assert notes[0].duration == 384
+
+
+def test_pyxel_tie_note():
+    source = "0: T120 L4 O4\n  C4 & C4"
+    tokens = tokenize(source, "pyxel")
+    ns, errors = parse_pyxel(source, tokens)
+    assert not any(e.severity == "error" for e in errors)
+    notes = [e for e in ns.channels["Pulse1"].events if isinstance(e, NoteEvent)]
+    # C4 & C4 should produce a single note with doubled duration (192*2=384)
+    assert len(notes) == 1
+    assert notes[0].duration == 384
 
 
 def test_pyxel_tie_length():