feat/progression-picker-and-loop-polish

[TwDover]

feat: progression picker, loop playback, idiomatic non-4/4 drums

Phase 4 feature work + fixes surfaced during ear-validation.

Progression picker

• Unified "Progression" control in the setup form: style default (auto),
  the style's own templates (roman numerals + resolved chords), a saved
  library, or free-text custom. Consolidates the old Advanced-only field.
• Saved progressions are their own localStorage library, reusable across
  any style/key. Style switch resets a style-template pick; keeps custom/saved.
• Backend: list_styles emits progression_templates (deduped, capped);
  StyleInfo schema exposes it so /styles serves it.

Loop playback

• Loop-mode clips now auto-loop on play; songs/arrangements play through.
  toggle() takes a loop hint threaded via ExportPanel (mode) and SongResult.

Non-4/4 drums

• Idiomatic compound/odd feel: snare backbone on felt pulses, compound
  meters ride an eighth grid grouped in threes. 4/4 stays byte-identical.
• Fix: loop routes now pass meter to write_midi/write_combined_midi, so
  the .mid time_signature meta matches the request instead of always 4/4.

Priors toggle

• The "Use my local MIDI corpus" toggle always renders now (disabled with
  a hint when no prior exists) instead of self-hiding.

Tests: backend 222, frontend 123. Roadmap updated.