fix: match a cuesheet's filename however it is capitalised

[casmith]

Three Final Fantasy VII discs failed to install:

Failed  convert Final Fantasy VII Disc 1.cue: invalid cuesheet:
        Final Fantasy VII Disc 1.cue references FINAL FANTASY VII DISC 1.BIN, which is missing

against a file plainly sitting right there. The archive holds:

Final Fantasy VII [Disc1of3] [U] [SCUS-94163]/Final Fantasy VII Disc 1.bin
Final Fantasy VII [Disc1of3] [U] [SCUS-94163]/Final Fantasy VII Disc 1.cue

and the sheet — a 2003 rip — says:

FILE "FINAL FANTASY VII DISC 1.BIN" BINARY
  TRACK 01 MODE2/2352
    INDEX 01 00:00:00

Cuesheets are routinely written on Windows, where the case of a filename carries no information. The rip is fine; it works everywhere except a case-sensitive filesystem.

Change

A FILE reference now falls back to a case-insensitive match within the sheet's own directory.

• The exact name always wins, and the directory is only read when the exact name doesn't resolve — so nothing gets slower, and no ambiguity is introduced where the filesystem itself has none.
• A name matching nothing is returned unchanged, so the error still quotes what the sheet actually said.
• BinName keeps the name as written: that is what an error should quote and what an archive listing agrees with. Only the resolved path changes.

A quieter symptom it also fixes

The scanner excludes the tracks a cuesheet names, so that one rip is one title rather than several files. That exclusion is by path — so a sheet whose case didn't match left its .bin unexcluded and listed as a title of its own, beside the game it belongs to.

Scope

Only the on-disk path was affected. Inside an archive the data track is chosen from the listing rather than by name, which is why these titles scanned correctly and only failed at conversion.

Tests

• TestParseCueFileMatchesTheFilenameCaseInsensitively uses the real sheet's exact contents and filename. Mutation-checked: removing the fallback compiles and fails it with the user's error verbatim — references FINAL FANTASY VII DISC 1.BIN, which is missing.
• TestParseCueFilePrefersTheExactFilename — two files differing only in case still resolve to the one named.
• TestParseCueFileStillReportsAMissingTrack — a genuinely absent file is still reported, quoting the sheet's name.

Also checked against the real cuesheet extracted from the archive: FINAL FANTASY VII DISC 1.BIN resolves to Final Fantasy VII Disc 1.bin and validates.

Full suite, go vet, gofmt and scripts/demo-smoke.sh pass.

https://claude.ai/code/session_018eBAB3V9GRxNwgpQcpGje4