perf(detection): vectorize box_iou_batch_with_jaccard

[RubenHaisma]

Description

box_iou_batch_with_jaccard computed COCO-style Jaccard IoU with a double Python for loop calling a scalar _jaccard helper once per (detection, ground-truth) pair — an O(N·M) per-element pattern in otherwise pure-NumPy code, against AGENTS.md's "vectorized throughout — no Python loops in hot paths" guideline. It is the inner IoU of COCOEvaluator._compute_iou (called once per (image, category) during mAP evaluation) and is also public API (sv.box_iou_batch_with_jaccard).

This replaces the loop with a broadcasted NumPy implementation and drops the now-unused scalar _jaccard.

Correctness (bit-identical)

The far corners are built as x2 = x + w and the union associated as (area_det + area_gt - area_inter) + eps, so the output is bit-identical to the previous per-pair result — verified to max|diff| = 0.0 over 4000 randomized trials including zero/negative-width degenerate boxes and crowd flags. Crowd semantics are preserved (a crowd ground truth uses the detection area as the union). End-to-end COCO mAP results are unchanged: the existing tests/metrics/ suite passes without modification, and the docstring doctest output is unchanged.

Performance

Speedup scales with batch size and is faster even at the smallest sizes (no regression regime):

batch (gt × dt)	loop	vectorized	speedup
5 × 5	26 µs	16 µs	~1.6×
15 × 60	876 µs	33 µs	~27×
50 × 100	4499 µs	68 µs	~66×

Tests

Adds TestBoxIouBatchWithJaccard: parity against an independent per-pair reference across empty / single / busy / degenerate+crowd batches, the crowd union semantics, the empty-input contract, and the is_crowd length guard. ruff check/format clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82%. Comparing base (09b2199) to head (10fef50).

❌ Your project check has failed because the head coverage (82%) is below the target coverage (95%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2359   +/-   ##
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  Coverage       82%     82%           
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  Files           68      68           
  Lines         9560    9556    -4     
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- Hits          7881    7878    -3     
+ Misses        1679    1678    -1     

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