feat/time-signature-foundation

[TwDover]

Non-4/4 time signatures: Meter model + generation-core threading (milestone 1)

Foundation for 3/4, 6/8, 7/8 (and other meters). This milestone makes the
whole pipeline meter-aware while keeping 4/4 byte-identical; the per-generator
within-bar feel for non-4/4 is milestone 2 (see docs/roadmap.md).

• core/meter.py: a Meter model centralizing the math — bar_beats (bar length
  in quarter-beats: 4/4->4.0, 3/4->3.0, 6/8->3.0, 7/8->3.5), compound/odd
  detection, and pulse positions. All positions stay in quarter-beats, so the
  one thing that varies is bar_beats and every bars * 4 becomes
  bars * bar_beats (== 4.0 for 4/4, hence unchanged).
• schemas: time_signature ("N/D", default "4/4") on GenerateRequest,
  BuildSongRequest, and RegeneratePartRequest; bad input falls back to 4/4.
• midi_writer: emits the correct time_signature meta for every meter
  (compound meters click on the dotted quarter); 4/4 meta unchanged.
• Threaded the meter through the generation core: all 7 generators (which
  already used a local beats_per_bar), _plan_sections, and the section loop.

4/4 stays byte-identical — 135 backend tests green, ruff clean. Non-4/4 runs
end-to-end with correctly-sized bars and MIDI meta.

Known limitation (milestone 2): generators still place some notes at fixed
within-bar offsets that overflow shorter bars (walking-bass beat 4, the drum
16-step grid), so non-4/4 output is structurally correct but not yet
rhythmically finished. No UI selector yet, so 4/4 is the only reachable meter
from the app — this milestone is safe to ship.