music realtime: CoreMIDI steering for Magenta RT2 (OP-1 and friends)

[a-github-name]

Feature

Live MIDI control for music realtime on macOS:

• --list-midi-inputs — enumerate CoreMIDI sources (name | manufacturer | model | unique ID)
• --midi-input <name-or-id> — connect a source; exact-ID and exact-name matches win, substring matches must be unambiguous
• --midi-channel all|1-16, --midi-note-offset <n>
• repeatable --midi-cc cc=target:min:max mappings (temp, topk, mc, notes, drums, drumless, unmask, seed, onset)

Note-on/off and mapped CCs feed the same MagentaRT2LiveControlQueue as interactive stdin steering (the former private interactive-controls class, extracted to Support/ and shared by both paths). Prompt changes stay on stdin — the prompt encoder swap can stall a frame, so MIDI is for notes and continuous controls (matching the engine constraints established in #132).

Review hardening on top of the initial implementation

• Fixed a memory-safety bug in packet iteration: the original loop copied MIDIPacket into a local and called MIDIPacketNext on the copy — that walks past the copy's stack storage and reads garbage whenever a MIDIPacketList carries more than one packet (chords, CC sweeps, clock+note bursts). Now uses MIDIPacketList.unsafeSequence() and reads the true variable-length payload via the data field offset.
• Running status support: the original parser treated every data byte outside a leading status byte as stray and dropped it — under running status (one status byte, many data pairs, common from hardware) every message after the first was lost. Parsing is now a small stateful MIDI 1.0 parser (MagentaRT2MIDIStreamParser) that keeps running status across packet boundaries, tolerates system-realtime bytes (clock, active sensing) interleaved mid-message, advances correctly past pitch-bend/program-change, and normalizes velocity-0 note-on to note-off per spec.
• Testability: the parser is pure and platform-independent, so the protocol edge cases above are unit-tested directly (multi-message packets, running-status chords, split messages, SysEx + stray-byte recovery).
• Ambiguous source matches fail loudly with the candidate list instead of silently binding the first substring hit.
• Minor: stdin reader task only spawns in interactive mode; snapshot() drops an unused parameter.

Validation

• swift test --filter MusicGenerateCommandParsingTests: 19 tests, 0 failures (7 new: 3 CLI/mapping parsing from the original work, 4 parser protocol tests from review).
• Flag surface documented in docs/cli.md, docs/runtime/music.md, the music-generate guide, README example, and CHANGELOG.
• Live end-to-end (OP-1 in hand) remains the promotion gate per the MagentaRT2: opt-in non-blocking prompt swaps, 1ms prompt-wait poll #128/MagentaRT2: root-cause engine metallib failure (LFS), drop unsafe swap option #132 pattern — the engine-side realtime loop is unchanged by this PR.

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