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, 7 insertions, 2 deletions

diff --git a/tests/test_server.py b/tests/test_server.py
index 533c88b..f858e0f 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -1,6 +1,10 @@
 """Tests for MCP server compose_mml tool."""
 
-from mml_composemusic_mcp.server import compose_mml
+from mml_composemusic_mcp.server import _execute_compose_mml
+
+
+def compose_mml(**kwargs):
+    return _execute_compose_mml(**kwargs).result
 
 PPMCK_MML = """#TITLE "Test"
 A t120 l4 o4 v15 q2
@@ -11,7 +15,8 @@ A t120 l4 o4 v15 q2
 def test_compose_ppmck():
     result = compose_mml(action="compose", mml=PPMCK_MML, mode="ppmck")
     assert result["success"] is True
-    assert result["wav_path"] is not None
+    assert result["audio"]["mime_type"] == "audio/wav"
+    assert "wav_path" not in result
     assert result["duration_sec"] > 0