feat/phase4-features-and-polish

[TwDover]

Phase 4 — features & polish

Completes Phase 4 of the roadmap, plus two UX fixes surfaced during
ear-validation. All backend (223) and frontend (123) tests green; 4/4 and
default-song output stay byte-identical.

Tempo automation (roadmap item)

• New tempo_automation knob (0–1) scales the song tempo map — chorus push,
  pre-chorus lean, ending ritardando — via m = intensity/0.5: 0 = flat,
  0.5 = the classic subtle default (byte-identical), 1 = expressive.
• "Tempo motion" control (Off / Subtle / Expressive) on the song form; threaded
  through build + melody-upload + every regen/replay path and persisted in
  song_meta.json so re-rolls stay sample-locked. (Mid-song key modulation —
  chorus/bridge/final-chorus shifts — was already shipped.)

Progression picker (bonus)

• Unified "Progression" control: style default (auto), the style's own templates
  (roman numerals + resolved chords), a saved library, or free-text custom —
  consolidating the old Advanced-only field.
• Saved progressions are their own localStorage library, reusable across any
  style/key. list_styles/StyleInfo now expose progression_templates.

Surface WAV/offline export (roadmap item)

• Split the crowded action row into a labelled Export group separating Audio
  (⏬ WAV / ⏬ Stems (WAV), accent-styled) from MIDI (ZIP downloads), so
  audio export reads as a first-class action instead of a cryptic ⚡.

Loop playback (bonus)

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

Idiomatic non-4/4 drums + meta fix

• 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 (roadmap item)

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