feat(gp2rs): emit lyrics + vocal_pitch sidecars from GP vocal tracks

GP files with a vocal track previously produced only the legacy vocals
arrangement XML; the melody sidecar converter existed but was never
wired, so imported paks had no karaoke-ready pitch data.

- convert_file() now calls _emit_vocal_sidecars() after writing the
  vocals XML (best-effort: a sidecar failure never breaks conversion,
  matching the notation-sidecar posture)
- lyrics sidecar is projected from the just-built vocals XML so timing
  stays in lockstep; the XML lyric convention's trailing '+' (join) is
  mapped to the spec 7.1 '-' (join) - a passthrough would have turned
  every joined word into a line break
- convert_vocal_track_to_pitch_sidecar() gains require_lyric=False for
  lyric-less melody tracks (pitch sidecar only)
- attach_vocal_sidecars_to_sloppak() added as the assembly-step helper
  (companion to attach_notation_to_sloppak): writes the sidecars into a
  directory-form pak and sets manifest lyrics/lyrics_source/vocal_pitch
  with no-clobber guards
- gp2rs_gpx registered in the spec-conformance READERS list

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: ChrisBeWithYou <chris@rifflarr.local>

tools/check_spec_conformance.py

@@ -58,6 +58,7 @@ READERS = [
     "lib/enrichment.py",
     "lib/songmeta.py",
     "lib/gp2notation.py",        # rewrites manifest.yaml; stamps feedpak_version
+    "lib/gp2rs_gpx.py",          # attach_vocal_sidecars_to_sloppak: lyrics/vocal_pitch keys
     "lib/routers/ws_highway.py", # reads `authors` off a feedpak manifest
     "lib/routers/chart.py",      # Get-info panel: binds `m = load_manifest(...)`
     "lib/routers/song.py",       # enrichment gap-fill: reads the manifest directly