feat: stream archive to stdout when compress output is -

[Mathjk]

Refs #1092. Complements #110 (which covers - = stdin for decompress; this is the compress-side twin).

Bug

ouch compress <files> - created a literal file named - on disk (or, without --format, a confusing "Cannot compress to -" error hinting at -.tar.gz).

Fix — stream to stdout, --format required

Since stdout has no extension to detect a format from, - requires --format — mirroring ouch decompress -, which already reads stdin on - and likewise requires --format (precedent also in #687).

• commands/mod.rs: on is_path_stdin(&output_path) the output writer is io::stdout(); conflict resolution, the size report, and partial-file cleanup are skipped for the stdout target.
• commands/compress.rs: compress_files takes impl Write instead of fs::File (wrapped in the existing BufWriter).
• check.rs: dedicated error — Cannot compress to "-" (stdout) + --format hint — replaces the nonsense -.tar.gz suggestion.
• All diagnostics already go to stderr, so the piped stream stays clean.
• args.rs help documents - → stdout; tests/integration.rs gains a compress_to_stdout test (error without --format, tar stream, no literal - file, decompress roundtrip).

Verified

• ouch compress a.txt - --format tar | wc -c → 2048; file says POSIX tar archive
• compress a.txt sub - --format tar > out.tar → tar tf lists entries; INFO logs on stderr
• Roundtrip: compress … - --format tar.gz | decompress - --format tar.gz -d out → files extracted
• zip/gz stream correctly; normal compress f out.zip unchanged
• | head -c 100 → quiet truncation, exit 0

cargo test all green (53 unit + 58 integration + ui/mime); fmt/clippy -D warnings clean (default + --no-default-features).

Open to maintainer choice on whether bare - could default to a format instead of requiring the flag — easy to relax.

[valoq]

• mod.rs:106: there is no terminal check and binary data is always written to a tty
• compress.rs:208: only tar flushes (line 158). For other formats, final write errors are lost. This bug already existed but stdout output makes it common.

[Mathjk]

Addressed in 6cbdf71. compress - on a tty now errors out with a hint; redirected output still works. A writer.flush()? after the match covers all formats — final write errors propagate instead of being dropped. Known limit: gzp's flush never reaches the sink, so its error surfaces as a panic in Drop rather than a clean Err; a real fix needs per-format finish(), happy to do it if you want it in this PR. Tests added.

[valoq]

With output "-" a file that stdout is redirected to inside the compressed directory is not skipped