feat: return composed WAV as MCP audio content

Replace the server-local wav_path result with a protocol-native multimedia response.

- Split internal composition execution from MCP ToolResult serialization so filesystem paths never enter public payloads.

- Return every successfully composed WAV as AudioContent with the audio/wav MIME type, regardless of file size.

- Keep structured audio metadata, duration, note sequence, and validation details alongside the multimedia content.

- Use CallToolResult.isError as the authoritative MCP execution status while retaining compose success and validate valid as domain fields.

- Treat invalid compose/API requests and runtime failures as tool errors, while invalid MML discovered by validate remains a successful validation operation.

- Sanitize synthesis, file-write, audio-serialization, and unexpected failure responses so internal paths and exception details are not exposed.

- Add protocol-level coverage for base64 round trips, content block types, error semantics, ResourceLink absence, and path non-disclosure.

- Update existing tests for the separated execution layer and path-free result contract.

- Document AudioContent and isError behavior in the README, MCP schema, and design specification.

- Require the tested FastMCP 3.4.4 API line and exclude the incompatible 4.x major version.

Verification: 487 tests passed; ruff check passed; git diff --check passed; uv lock --check passed; and a manual MCP call returned TextContent plus AudioContent with no wav_path.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/mml_composemusic_mcp/synthesizer.py b/src/mml_composemusic_mcp/synthesizer.py
index a289a1e..7085342 100644
--- a/src/mml_composemusic_mcp/synthesizer.py
+++ b/src/mml_composemusic_mcp/synthesizer.py
@@ -545,14 +545,14 @@ def write_wav(path: Path, wave_data: np.ndarray, sample_rate: int) -> list[Error
             wav.setsampwidth(2)
             wav.setframerate(sample_rate)
             wav.writeframes(pcm.tobytes())
-    except Exception as exc:
+    except Exception:
         errors.append(
             ErrorDetail(
                 code=ErrorCode.RUNTIME_WAV_WRITE_FAILED,
                 phase=ErrorPhase.RUNTIME,
                 line=0,
                 column=0,
-                message=f"WAVファイルの出力に失敗しました: {exc}",
+                message="WAVファイルの出力に失敗しました。",
                 severity="error",
                 hint="出力先を確認してください。",
             )