[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:

Run any input with CEA-708 captions (e.g. sample c83f765c661595e1bfa4750756a54c006c6f2c697a436bc0726986f71f0706cd.ts) with -o /dev/null. The 708 service file becomes /dev/null.p1.svc01.srt, which can't be created, and the run aborts with SIGABRT (exit 134): panicked at src/decoder/service_decoder.rs:897. Same for any unwritable per-service path (e.g. a directory named out.p1.svc01.srt next to -o out.srt).

---

screen_print() unwrapped the result of writer_output(), so a CEA-708 output file that couldn't be created panicked. The C writer (ccx_decoders_708_output.c) and v0.94 before the port treat a failed open as a fatal file-creation error, exit 5, like the main output path does. This does the same, and includes the file name in the error, which previously carried only the OS message.

Case	master	this PR
-o /dev/null	SIGABRT (134)	exit 5, [CEA-708] Failed to open output file: /dev/null.p1.svc01.srt: Permission denied (os error 13)
directory in place of the service file	SIGABRT (134)	exit 5, [CEA-708] Failed to open output file: blk.p1.svc01.srt: Is a directory (os error 21)

Verified:

• New test: a file that can't be created yields an error naming it; it fails without the change.
• All 23 local CEA-708 samples: output and exit codes byte-identical to master.
• cargo test, cargo fmt --check and cargo clippy -D warnings clean in both crates.