fix(midi): always export SMF as Type 1 (Parallel)

[PhysShell]

Problem

A generated single-track riff (griff generate …) produced a MIDI file that
strict readers reject with "a type 0 file must contain exactly one track".
Multi-track output (import, complement) opened fine.

Root cause

export_score (core/src/midi.rs) chose the SMF header format from the note-track
count:

let format = if score.tracks.len() == 1 { Format::SingleTrack } else { Format::Parallel };

But a dedicated meta/conductor chunk is always emitted (ADR-0003). So a
single-track score serialises to two chunks (meta + 1 note) while declaring
Type 0 — and SMF Type 0 must contain exactly one track chunk. The file is
therefore spec-invalid. Multi-track scores avoided this only by accident of
already having ≥2 note tracks.

Confirmed on a freshly generated riff: header was format=0, ntracks=2 (invalid);
after the fix it is format=1, ntracks=2 with two real MTrk chunks.

Fix

Always declare Format::Parallel. One line, plus a comment explaining the
invariant.

Test (TDD, red → green)

export_score_single_track_is_type1_parallel: a single-track score must export
as Format::Parallel with ≥2 chunks. Red before the fix (was Type 0), green
after. Full griff-core suite (87 tests) and clippy --all-targets are clean.

This is the first of two fixes for the reported import/export issues; the GP6
string-indexing pitch fix follows in a separate PR.

https://claude.ai/code/session_01TTUbGjzD8ysnVnCJnZJE95

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Generated by Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Fixed MIDI file export to consistently use the proper header format for all score configurations, ensuring accurate metadata inclusion and correct file structure compatibility.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: ce3ba47e-6f6c-4c3e-9cf1-c7cb34eb6cd8

📥 Commits

Reviewing files that changed from the base of the PR and between 44c3cc7 and 8047a0f.

📒 Files selected for processing (1)

• core/src/midi.rs

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📝 Walkthrough

Walkthrough

export_score in core/src/midi.rs is changed to unconditionally set the SMF header format to Format::Parallel instead of conditionally using Format::SingleTrack for single-track scores. A regression test is added that exports a single-track fixture and asserts the resulting SMF has Format::Parallel and at least two tracks.

Changes

SMF Header Format Fix

Layer / File(s)	Summary
Unconditional Parallel format + regression test core/src/midi.rs	export_score removes the conditional Format::SingleTrack branch and always emits Format::Parallel. Test imports gain midly::Format, and a new regression test exports a single-track fixture, then asserts the parsed SMF reports Format::Parallel and contains at least two chunks.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐇 A track once lonely, declared too slim,
Now marches in parallel, row by row!
The meta chunk leads, the notes follow trim,
Type-1 format puts on quite the show.
No more single-file fib — the header's true,
With two chunks at least, the spec shines through! 🎵

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically summarizes the main change: fixing MIDI export to always use SMF Type 1 (Parallel) format instead of conditionally selecting Type 0 based on track count.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/midi-format-type1

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