fix(chimeric): create output parent directory before writing

[pinin4fjords]

Summary

rustar-aligner --chimSegmentMin > 0 combined with --twopassMode Basic aborts the run when --outFileNamePrefix doesn't end in /:

[INFO  rustar_aligner] Chimeric detection enabled (chimSegmentMin=12)
Error: I/O error: No such file or directory (os error 2) (SAMPLE./Chimeric.out.junction)

The chim writer constructs its path as <prefix>/Chimeric.out.junction (treating --outFileNamePrefix as a directory, per the existing behaviour tracked in #26). In two-pass mode the chim writer fires before any other output writer creates that directory, so the File::create call fails. Without two-pass mode the bug is masked because another output writer incidentally creates the dir first.

Fix

Call std::fs::create_dir_all on the parent of the chim output path before opening the file. Minimal, scoped fix - does not change the existing prefix-as-dir behaviour (that's left to #26).

Test plan

• New unit test asserts the chim writer succeeds when the parent dir doesn't exist
• Existing tests pass
• cargo build
• cargo clippy --all-targets -- -D warnings
• cargo fmt --check

After the fix, rustar-aligner --chimSegmentMin 12 --twopassMode Basic --outFileNamePrefix sample. ... completes and writes sample./Chimeric.out.junction (with 14-column content matching STAR's format).

Fixes #35

[pinin4fjords]

Verified the fix end-to-end on macOS/aarch64 with the binary built from 8589859.

Ran the exact failure mode from the issue (PE + --twopassMode Basic + --chimSegmentMin 12 + bare-dot --outFileNamePrefix RUS.) against the yeast test inputs:

[INFO  rustar_aligner] Writing pass 1 junctions to RUS./SJ.pass1.out.tab
[INFO  rustar_aligner] Pass 1 discovered 16 novel junctions
...
[INFO  rustar_aligner::stats] Total mapped: 42631 (85.26%)
[INFO  rustar_aligner] Wrote RUS./Log.final.out
[INFO  rustar_aligner] Alignment complete!

$ ls RUS./
Aligned.out.bam
Aligned.toTranscriptome.out.bam
Chimeric.out.junction
Log.final.out
SJ.out.tab
SJ.pass1.out.tab

Pre-fix the same invocation crashed before any output was written:

[INFO  rustar_aligner] Chimeric detection enabled (chimSegmentMin=12)
Error: I/O error: No such file or directory (os error 2) (RUS./Chimeric.out.junction)

After the fix the run completes cleanly, the parent dir is created by create_dir_all, and Chimeric.out.junction lands alongside the other outputs. LGTM from the consumer side.