perf(coco): fuse load_res_anns into one validate/derive/id pass

[Borda]

This pull request significantly improves the performance and clarity of the load_res_anns method in the COCO Rust implementation by validating, deriving geometry, and assigning IDs in a single pass over the detections. It also introduces new regression tests to ensure correct warning behavior and removes unnecessary code. These changes lead to a measurable speedup for large workloads and maintain bit-identical evaluation results.

Performance and Validation Improvements

• The load_res_anns function now validates, derives geometry, and assigns IDs in a single loop over the detection annotations, instead of five separate passes. It checks ground-truth (GT) membership using the existing index rather than rebuilding a HashSet each time, resulting in a 16–19% speedup on large workloads. The method also ensures only one warning per mismatch kind (image/category), naming the first offender, and still immediately rejects NaN scores. [1] [2] [3]

Codebase Simplification

• The now-redundant validate_results method has been removed, with its logic fused into the main loop of load_res_anns. This reduces code duplication and improves maintainability.

Testing Enhancements

• Two new regression tests have been added:
  • load_res_anns_warns_once_per_mismatch_kind ensures that only one warning is issued per mismatch type and that the first offending annotation is reported.
  • load_res_anns_skips_category_check_when_gt_has_no_categories ensures that no category mismatch warnings are emitted if the GT set has no categories.

Minor Cleanups

• Unused imports (HashSet) have been removed from coco.rs.
• Minor formatting and documentation improvements in the changelog for clarity.

These changes collectively make the code faster, clearer, and better tested, especially for large-scale detection evaluation workloads.

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• validate_results, geometry derive, and id assignment merged into a single loop over detections instead of five separate passes
• image_id/category_id GT membership now probes self.imgs/self.cats (the index this COCO already built) instead of rebuilding a fresh HashSet of GT ids on every load_res_anns call
• NaN score rejection, derive_from_bbox's rectangular polygon, the unconditional id reassignment, and once-per-kind mismatch warnings are all preserved in content, only fused into fewer passes
• both the in-memory dict path and the numpy loadRes(ndarray) fast path share load_res_anns, so both benefit from one fix
• measured on a 1.5M-annotation RF-DETR-shaped workload: loadRes -16% to -19% (non-overlapping ranges across two rep-count runs); end-to-end effect not distinguishable from run-to-run noise
• 10/10 bit-identical vs main across ten configs
• new tests load_res_anns_warns_once_per_mismatch_kind and load_res_anns_skips_category_check_when_gt_has_no_categories fail on 3 injected defects (dropped warn-once guard on each mismatch kind, dropped has_cats gate)