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, 14 insertions, 18 deletions

diff --git a/tests/test_compose_mml_tool.py b/tests/test_compose_mml_tool.py
index c33e737..4f904e4 100644
--- a/tests/test_compose_mml_tool.py
+++ b/tests/test_compose_mml_tool.py
@@ -1,7 +1,5 @@
 """Exhaustive tests for the compose_mml MCP tool."""
 
-from pathlib import Path
-
 VALID_PPMCK = """#TITLE "Test"
 A t120 l4 o4 v15 q2
   c e g c
@@ -27,8 +25,10 @@ VALID_PYXEL = """0: T120 L4 O4 V100 @1
 def test_compose_ppmck(compose_mml, tmp_output_dir):
     result = compose_mml(action="compose", mml=VALID_PPMCK, mode="ppmck")
     assert result["success"] is True
-    assert result["wav_path"] is not None
-    assert Path(result["wav_path"]).exists()
+    assert result["audio"]["mime_type"] == "audio/wav"
+    assert result["audio"]["size_bytes"] > 44
+    assert "wav_path" not in result
+    assert list(tmp_output_dir.rglob("output.wav"))
     assert result["duration_sec"] > 0
     assert result["note_sequence"] is not None
     assert "validation" in result
@@ -39,8 +39,9 @@ def test_compose_ppmck(compose_mml, tmp_output_dir):
 def test_compose_pyxel(compose_mml, tmp_output_dir):
     result = compose_mml(action="compose", mml=VALID_PYXEL, mode="pyxel")
     assert result["success"] is True
-    assert result["wav_path"] is not None
-    assert Path(result["wav_path"]).exists()
+    assert result["audio"]["mime_type"] == "audio/wav"
+    assert "wav_path" not in result
+    assert list(tmp_output_dir.rglob("output.wav"))
     assert result["duration_sec"] > 0
 
 
@@ -95,7 +96,7 @@ def test_synthesis_templates_compose(compose_mml, tmp_output_dir):
                 f"{mode}/{template} synthesis failed: {result['validation']}"
             )
             assert result["duration_sec"] > 0
-            assert Path(result["wav_path"]).exists()
+            assert result["audio"]["size_bytes"] > 44
 
 
 def test_template_invalid_fallback(compose_mml):
@@ -131,7 +132,8 @@ def test_compose_unknown_mode(compose_mml):
 def test_compose_with_syntax_error(compose_mml):
     result = compose_mml(action="compose", mml="A t0\n  c", mode="ppmck")
     assert result["success"] is False
-    assert result["wav_path"] is None
+    assert result["audio"] is None
+    assert "wav_path" not in result
     assert result["duration_sec"] == 0
     assert any(e["severity"] == "error" for e in result["validation"]["errors"])
 
@@ -168,8 +170,7 @@ def test_compose_normalize(compose_mml, tmp_output_dir):
     on = compose_mml(action="compose", mml=base, mode="pyxel", normalize=True)
     off = compose_mml(action="compose", mml=base, mode="pyxel", normalize=False)
     assert on["success"] and off["success"]
-    assert Path(on["wav_path"]).exists()
-    assert Path(off["wav_path"]).exists()
+    assert len(list(tmp_output_dir.rglob("output.wav"))) == 2
 
 
 def test_compose_creates_timestamped_output_directory(compose_mml, tmp_output_dir):
@@ -182,14 +183,9 @@ def test_compose_creates_timestamped_output_directory(compose_mml, tmp_output_di
     assert result1["success"] is True
     assert result2["success"] is True
 
-    path1 = result1["wav_path"]
-    path2 = result2["wav_path"]
-    assert path1 is not None
-    assert path2 is not None
-    assert path1 != path2, "Two compose calls produced the same output directory"
-
-    p1 = Path(path1)
-    p2 = Path(path2)
+    paths = sorted(tmp_output_dir.rglob("output.wav"))
+    assert len(paths) == 2
+    p1, p2 = paths
     assert p1.exists()
     assert p2.exists()
     assert p1.name == "output.wav"