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.

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