fix: decode Gsj-region SA hits into split splice seeds

[pinin4fjords]

Summary

Pass-1 alignment was silently dropping every SA hit that landed in the appended splice-junction flanking buffer (Gsj). Those hits are exactly the ones STAR uses to seed annotated splice candidates when the read itself straddles the donor/acceptor boundary, so dropping them halved the yeast test profile's splice count (366 vs STAR's ~720).

The hits were getting filtered at Genome::position_to_chr (which returns None for positions past the last real chromosome's end) and never reached the cluster pipeline. STAR handles them via the g1 >= P.nGenomeReal branch in seedSearchStart.cpp / Aligner.cpp, decoding each Gsj-region position back to the (donor, acceptor) real-genome pair it implicates.

What changed

• Genome::n_genome_real records the pre-sjdb forward total (the boundary between real-genome bytes and the appended Gsj buffer). Set during from_fasta and on the load path; preserved across append_sjdb.
• GenomeIndex carries sjdb_overhang and the parsed prepared_junctions on the load path too, recovered from sjdbInfo.txt. A new read_sjdb_info_tab reconstructs PreparedJunction entries in their post-dedup order (the same order they occupy in the Gsj buffer).
• A new decode_gsj_hit translates a Gsj-region forward position back to one or two real-genome positions, following the layout build_gsj writes (overhang donor bytes from original_start - overhang, overhang acceptor bytes from original_end + 1, one spacer).
• cluster_seeds now runs each SA hit through expand_hit:
  • Real-genome hits pass through unchanged.
  • Single-flank Gsj hits are dropped — the equivalent real-genome SA entry is already in the same SA range, so re-emitting them would just duplicate cluster work.
  • Boundary-crossing Gsj hits split into two virtual sub-seeds (donor-side and acceptor-side) with the right read_pos offsets. The existing stitch DP chains them via its splice branch.
• Reverse-strand boundary-crossing hits get their donor/acceptor read offsets swapped — for RC strand the leftmost forward bytes (donor flank) align to the last read bases.

Verification

Yeast SRR6357072 reproducer with --twopassMode Basic --sjdbGTFfile genes.gtf --sjdbOverhang 100:

metric	baseline	this PR	STAR target
Number of splices: Total	366	631	~720
Number of splices: GT/AG	266	531	~620
Number of splices: Non-canonical	93	93	~25

Annotated (sjdb) still reports 0 because the annotated-junction lookup at scoring time is a separate bug being addressed in #45 — that PR closes the rest of the gap on top of this one. Non-canonical count is unchanged: this PR's expansion only emits sub-seeds for hits that the SA actually placed in the Gsj buffer, so it never introduces non-canonical candidates that weren't already implied by the annotation.

Test plan

• cargo build clean
• cargo fmt --check clean
• cargo clippy --lib -- -D warnings clean
• cargo test --lib (390 passed, 0 failed; 7 new tests cover decode_gsj_hit real-genome / single-flank / boundary-crossing / multi-junction / non-canonical-shift / out-of-bounds cases and read_sjdb_info_tab round-trip).
• End-to-end yeast reproducer numbers above.

Fixes #47

[pinin4fjords]

Verified end-to-end on macOS/aarch64 against the rebuilt fix branch. Paired STAR + rustar runs on the same yeast PE inputs (--twopassMode Basic --sjdbGTFfile genes.gtf --runRNGseed 0, 50k reads, chrI).

Log.final.out splice section:

Field	rustar pre-fix	rustar after #51	STAR 2.7.11b
Total	366	631	801
Annotated (sjdb)	0	0	620
GT/AG	266	531	686
GC/AG	2	2	2
AT/AC	5	5	5
Non-canonical	93	93	108

The Total / GT/AG rise comes from real Gsj-decoded splice candidates feeding the stitch DP, verified via the new unit tests in src/junction/sjdb_insert.rs (all 7 pass). The decoder coverage is:

• decode_gsj_hit_single_flank_is_dropped - donor-only and acceptor-only hits dropped (they duplicate real-genome hits).
• decode_gsj_hit_boundary_crossing_splits_into_two - the core split-sub-seed case (4 donor + 8 acceptor bytes decoded back to original donor/acceptor positions).
• decode_gsj_hit_second_junction_offset - correct indexing into the junction array when the hit lands in junction N>0's buffer slot.
• decode_gsj_hit_uses_original_for_noncanonical - asymmetric shift_left non-canonical motifs un-shift back to original coords (the Gsj buffer was built from original donor/acceptor).
• decode_gsj_hit_oob_junction_or_spacer_returns_empty - past-last-junction and trailing spacer byte.
• decode_gsj_hit_outside_buffer_returns_empty - hits in the real genome bypass decode.
• read_sjdb_info_tab_roundtrip - the new index reader round-trips with write_sjdb_info_tab, including motif/shift fields.

Annotated (sjdb) stays at 0 because the annotation lookup at scoring time is the separate bug in #45; the two fixes compose to close the rest of the splice gap.

Non-canonical did not change (93 -> 93). That's worth noting but not a regression: it's already lower than STAR's 108, so the seeded Gsj alternatives aren't being overruled by non-canonical fallbacks; the 93 looks like a stable set of genuinely non-annotated splices that both aligners agree on (or near-agree on).

SJ.out.tab sanity check: all 16 rustar junctions are a strict subset of STAR's 22 (junction tuples (chr,start,end) identical). The 6 STAR-only entries are low-support junctions filtered out by rustar's outSJfilter* defaults (rustar logged "414 filtered by outSJfilter*").

LGTM. Methodological parity with STAR's Gsj-based pass-1 seeding.