feat(gp_autosync): piecewise time-warp helpers + refine_sync onset pass (#787)

* feat(gp_autosync): piecewise time-warp helpers + refine_sync onset pass

auto_sync computes per-bar sync points but consumers only ever applied the
scalar bar-1 audio_offset, so any tempo drift between the recording and the
tab's authored tempo accumulated over the song. Add the librosa-free helpers
needed to apply the full piecewise mapping:

- bar_start_times(gp_path): per-bar score times sharing auto_sync's axis
  (GPIF bar-resolution map, GP3/4/5 per-tick integration)
- build_warp_anchors(points, bar_starts): monotonic (score, audio) anchors
- warp_time(t, anchors): piecewise-linear map with edge-slope extrapolation
- warp_song_times(song, warp): retime a lib.song.Song in place (notes,
  sustains, chords, beats, sections, anchors, handshapes, phrase levels,
  tone changes, tempo overrides)
- gp_has_expandable_repeats(gp_path): detects GP3/4/5 repeat/volta/direction
  markup whose playback expansion auto_sync's as-written points cannot map

Also implement refine_sync() — the editor's refine-sync endpoint has imported
it since the snapshot but it never existed in lib, so the Refine button 500'd.
It densifies the DTW points to every Nth bar and re-times each with a local
onset phase sweep (radius clamped under half a beat to avoid one-beat locks,
short scoring grid + median residual snap against the first beats). Synthetic
click-track validation: ~13ms mean / ~40ms max error from ±180ms coarse input
across 117-123 BPM recordings of a 120 BPM tab.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* review: Copilot round 1 — normalize bar_start_times GP3/4/5 parse failures to ValueError, document ImportError

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/gp2rs_gpx.py

@@ -1506,6 +1506,10 @@ def convert_file(
     # Surface that to the caller rather than only the docstring: if the score
     # actually uses repeats, the produced bar count/timing will differ from the
     # equivalent .gp5. Warn once so plugin code/logs don't silently drift.
+    # NB: lib/gp_autosync.gp_has_expandable_repeats() encodes this single-pass
+    # behaviour (.gp/.gpx never expand). Implementing GPIF expansion here MUST
+    # update that helper in the same change, or the editor's per-bar sync warp
+    # would silently retime repeated sections onto the wrong bars.
     if expand_repeats and any(
         mb.find('Repeat') is not None or mb.find('AlternateEndings') is not None
         for mb in masterbars