fix: off-by-one in bigWig interval start collapsed wide annot tracks (#233)

[d-laub]

Summary

Fixes #233. The Rust bigWig interval reader stored interval start - 1 instead of start. Since bigtools' get_interval already clips intervals to the query range, the -1 was a pure off-by-one. For intervals wider than the query region (e.g. binned/annotation bigWigs), the numba kernel intervals_to_tracks then computed a negative relative start and wrote an empty/truncated slice — collapsing wide intervals to value/span_length. Fine-grained tracks saw a one-base dilution. Also fixes a latent u32 underflow for contig-start (start = 0) intervals.

The fix is a single token in src/bigwig.rs: MaybeUninit::new(start - 1) → MaybeUninit::new(start). The same reader backs both per-sample tracks and annot_tracks, so both paths are corrected.

Changes

• src/bigwig.rs — store true interval start (the fix).
• tests/integration/tracks/test_annot_tracks.py — new fail-first regression test test_annot_bigwig_wide_intervals_full_width: builds 1 kb-binned intervals wider than the query regions, reads back the annot track, and asserts full-width fill (count_nonzero == e - s) plus value correctness against an independent pyBigWig.values() oracle. Failed 5-vs-350 on the buggy reader; passes now.
• tests/test_bigwig.rs — corrected two Rust unit-test expectations that had asserted the buggy start-1 coordinates ([0,5]→[1,5], [99,105]→[100,105]; the true clipped coordinates).
• tests/dataset/_snapshots/ — regenerated exactly the 4 track-dependent characterization snapshots (tracks_ragged, tracks_fixed, haps_tracks_fixed, haps_tracks_ragged); the change is a one-base edge correction. All seq-only snapshots unchanged.

Scope / constraints

• No public API signature/mode/default changes.
• No kernel change (_dataset/_intervals.py untouched) — minimal, parity-preserving.
• No SKILL.md change required (documents dataset-level reading, not the internal per-base coordinate).

Verification

• Full Python tree: 764 passed, 25 skipped, 4 xfailed.
• cargo test: 4/4 pass.
• ruff check + ruff format + typecheck: clean.

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