[FIX] Drop the previous file's bytes when opening a file

[cfsmp3]

In raising this pull request, I confirm the following (please check boxes):

Reason for this PR:

• This PR adds new functionality.
• This PR fixes a bug that I have personally experienced or that a real user has reported and for which a sample exists.
• This PR is porting code from C to Rust.

Sanity check:

• I have read and understood the contributors guide.
• I have checked that another pull request for this purpose does not exist.
• If the PR adds new functionality, I've added it to the changelog. If it's just a bug fix, I have NOT added it to the changelog.
• I am NOT adding new C code unless it's to fix an existing, reproducible bug.

Repro instructions:

Split any captioned TS at a 188-byte-aligned offset and pass both parts (binary concatenation is the default):
head -c $OFF in.ts > a.ts; tail -c +$((OFF+1)) in.ts > b.ts; ccextractor --autoprogram --out=srt a.ts b.ts -o split.srt
and compare with ccextractor --autoprogram --out=srt in.ts -o whole.srt. On master, captions after the join are repeated or mistimed (one 3 GB sample: every timestamp ~465 ms late), and some splits abort.

---

init_file_buffer(), called by every open(), rewound filebuffer_pos to 0 but kept bytesinbuffer when the buffer already existed. Whatever the previous input file left in the buffer — already consumed — therefore counted as unread data of the newly opened file. With binary concatenation, stream detection on the new file re-read the start of the previous file (its detection bytes, up to 1 MB) and fed it back into the stream at the join.

This resets bytesinbuffer too, in both the C (file_functions.c) and Rust (file.rs) versions; both are only called from the demuxer's open(). The existing Rust test that asserted the old behaviour now checks that the buffer is reused but both counters are reset; it fails without the fix.

Verified:

• 4 local TS samples × splits at 30/50/70%: master differs from the whole-file output in 11 of 12 (one aborts with SIGABRT); with this PR all 12 are byte-identical to the whole file, including the original 3 GB 50/50 case (608 and 708 outputs). Same with the C-only -min-rust build.
• 30 varied single-file samples: no output or exit-code changes.
• cargo test --lib (418), fmt and clippy clean; valgrind shows no invalid accesses on the concat path.