fix(gpx): clamp partial final BCFS sector so GP6 .gpx import works

Every real Guitar Pro 6 (.gpx) file failed to import with
"GPX BCFS sector pointer out of range (malformed file)".

A real .gpx's BCFZ-declared decompressed size isn't 0x1000-aligned, so
its last (small) container file lands in a partial trailing sector.
_parse_bcfs raised whenever a sector read would run past the buffer
end, rejecting the whole container before score.gpif could be extracted
-- so no GP6 file could be charted in the song editor. (GP7/GP8 .gp
files take the ZIP path, not BCFS, which is why this wasn't caught
earlier.)

Clamp the final sector read to the buffer end (the per-file size field
trims the padding anyway), matching canonical GPX readers (alphaTab /
PyGuitarPro). A sector whose start is past the end still raises, so the
malformed-file guard is preserved.

Verified against two real GP6 files -- both now unpack to valid GPIF
with all tracks. Adds the previously-missing positive BCFS round-trip
coverage: partial-final-sector, multi-file, sector-aligned baseline,
and the preserved out-of-range guard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/gp2rs_gpx.py

@@ -121,10 +121,18 @@ def _parse_bcfs(bcfs: bytes) -> dict:
             while sc <= max_sectors:
                 s = _gi(po + 4 * sc); sc += 1
                 if s == 0: break
-                so = s * SECTOR
-                if HDR + so + SECTOR > len(data):
+                start = HDR + s * SECTOR
+                # Real .gpx files' final sector is a few bytes short of a full
+                # 0x1000 block: the BCFZ-declared decompressed size isn't
+                # sector-aligned, so the last (small) container file lands in a
+                # partial trailing sector. Clamp the read to the buffer end —
+                # the per-file size field (`fs`, applied below) trims any
+                # padding — matching canonical GPX readers (alphaTab /
+                # PyGuitarPro slice-and-clamp). Only a sector whose *start* is
+                # past the end is genuinely malformed.
+                if start < 0 or start >= len(data):
                     raise ValueError("GPX BCFS sector pointer out of range (malformed file)")
-                fb.extend(data[HDR + so: HDR + so + SECTOR])
+                fb.extend(data[start: min(start + SECTOR, len(data))])
             else:
                 raise ValueError("GPX BCFS sector chain too long (malformed file)")
             files[fn] = bytes(fb[:fs])