STAR-compatible optional tags on unmapped records

[Psy-Fer]

fix: STAR-compatible optional tags on unmapped records + mismatch reason classification

Summary

Two related correctness fixes to unmapped-read handling that have no effect on alignment
decisions:

• Missing optional tags on unmapped records. STAR emits NH:i:0, HI:i:0, AS:i:0,
  nM:i:0, and uT:A: on every unmapped record. rustar-aligner emitted none of them.
  uT:A: in particular is parsed by MultiQC and similar QC tools to break down unmapping
  categories; without it those tools fall back to flag-only counting and lose the
  fine-grained breakdown.

• Mismatch-filtered reads misclassified as TooShort. When all transcript candidates
  were removed solely by the mismatch count/rate filter
  (--outFilterMismatchNmax / --outFilterMismatchNoverLmax), Log.final.out recorded
  them under "too short" instead of "too many mismatches", inflating the former and keeping
  the latter at zero. STAR distinguishes these two cases in
  ReadAlign_mappedFilter.cpp:20–30.

Closes the residual part of #48.

Changes

src/align/read_align.rs

Replaced the catch-all Some(UnmappedReason::TooShort) at the end of the quality-filter
block with logic that inspects the existing filter_reasons map: if only mismatch filters
fired (mismatch_max / mismatch_rate) the reason is TooManyMismatches; otherwise
TooShort.

src/io/sam.rs

• New free function insert_unmapped_tags(record, attrs, reason) inserts NH:i:0,
  HI:i:0, AS:i:0, nM:i:0 (gated on outSAMattributes as for mapped records) and
  uT:A: (always emitted, matching STAR's unconditional behaviour). uT:A: values:
  0=other, 1=too short, 2=too many mismatches, 3=too many loci.
• build_unmapped_record: rg_id: Option<&str> replaced by params: &Parameters +
  unmapped_reason: UnmappedReason; RG tag derived internally as other builders do.
• build_paired_unmapped_records: gains unmapped_reason: UnmappedReason parameter.
• build_half_mapped_records: unmapped-mate section calls insert_unmapped_tags with
  UnmappedReason::Other.

src/lib.rs

All four call sites updated. The now-unused rg_id_owned binding at the top of
run_single_pass removed.

src/io/bam.rs

Three test call sites updated to the new build_unmapped_record signature.

Test plan

• cargo test — 436 tests passing, 0 failures
• cargo clippy --all-targets — 0 warnings
• cargo fmt --check — clean
• New test test_unmapped_record_tags_emitted — verifies NH/HI/AS present and all
  four uT:A: values (0–3) on build_unmapped_record output
• New test test_unmapped_reason_mismatch_classification — verifies the
  filter_reasons → UnmappedReason mapping for all cases (mismatch-only, score-only,
  mixed, empty)
• SE benchmark: 8604/8926 (96.4%) — unchanged from post-batch-1 baseline (9-read
  drift is from batch 1's Gsj/sjdb changes, not this branch)
• PE benchmark: 8390/8390 both-mapped — exact match with STAR, unchanged