M9: machine interface — stdio MCP server, pinned Dockerfile, creator-facing README (+ CI sfizz build)

[jamiesun]

Summary

Outcome of the 2026-07 embedded-server audit: instead of a resident HTTP/API server, the third-party integration story ships as three thin artifacts, plus the CI sfizz fix this branch started with.

scorekit mcp — stdio MCP server (src/mcp.rs)

• Newline-delimited JSON-RPC 2.0 over stdin/stdout; exposes doctor, validate, schema, lint, build, diff as MCP tools.
• Pure protocol adapter: every tool call re-invokes the scorekit binary with --json; structured errors (code/field/exit_code) are passed through verbatim as isError: true results.
• Zero HTTP, zero auth, zero resident state — determinism and the "CLI is the machine interface" stance untouched.

Pinned deployment image (Dockerfile, .dockerignore)

• Multi-stage build (cargo build --locked → bookworm-slim), pinning FluidSynth 2.3.x, FFmpeg 5.1.x, and the SHA-256-verified MuseScore General SoundFont via the existing fetch script.
• scorekit doctor self-check at image build time; entrypoint is the CLI. HTTP gateways stay outside the image, owned by deployers.
• Verified: container build forest.yaml produces a seamless loop; docker run -i scorekit mcp completes the MCP handshake.

Docs

• New docs-site page Machine Interface: semver-stable contract (exit codes, --json error object, schema exports), Python/Node subprocess recipes, built-in MCP usage, Docker deployment.
• docs/roadmap.md: M9 milestone, capability-list entries, acceptance-matrix row for mcp (happy + failure path), audit outcome recorded (embedded HTTP server stays REJECTED; scorekit-core lib crate stays on HOLD pending a real consumer).
• README rewritten creator-first: inline first-track example, Agent workflow, humanize/grammar/SFZ sections; engineering detail moved to the docs site.

CI

• Build the sfizz renderer before running tests (original commit on this branch).

Acceptance matrix compliance

mcp is a new tier-1 feature and registers per rule 5: tests/cli.rs::mcp_initialize_lists_tools_and_validates_scene (happy path) and mcp_tool_failure_passes_structured_error_through (failure path: structured error passthrough, unknown tool → -32602, unknown method → -32601).

Validation

• cargo fmt + cargo clippy --all-targets clean
• 13 unit + 69 E2E tests pass
• mdBook builds
• Docker image builds and passes container smoke tests