docs(changelog): document cv2 fallback fixes from resolve pass

[resolve] PR #2439 — changelog entries for items 2,3 and the
copyMakeBorder scalar-channel bugfix, per AGENTS.md changelog policy.

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Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>

docs/changelog.md

@@ -19,6 +19,9 @@ date_modified: 2026-07-16
 
 ### Fixed
 - The cv2-free fallback now preserves OpenCV-compatible edge and keyword semantics for image borders, resizing, drawing, and small polygon masks, keeping ordinary production consumers usable without cv2.
+- The cv2-free fallback's `copyMakeBorder` now fills only channel 0 for a scalar border `value` on multichannel images, matching OpenCV's `Scalar(v)` semantics instead of broadcasting the value to every channel.
+- The cv2-free fallback's `addWeighted` now raises `ValueError` for a non-default `dtype` instead of silently ignoring it.
+- The cv2-free fallback's `approxPolyDP` now uses an O(N) farthest-point heuristic for closed contours instead of an O(N^2) full distance matrix, avoiding memory blowups on large contours.
 - Fixed [#2427](https://github.com/roboflow/supervision/issues/2427): size-bucketed `sv.Precision` and `sv.F1Score` no longer count out-of-bucket detections as false positives. `sv.Recall` now matches only targets in the requested bucket, and all three metrics prioritize in-bucket targets during matching, matching COCO evaluation and `sv.MeanAveragePrecision`. A pixel-perfect detector now scores 1.0 in every bucket.
 - `sv.hex_to_rgba` now rejects multiple leading `#` characters instead of silently normalizing them, matching `sv.is_valid_hex` and the documented single optional prefix.
 - `sv.box_iou_batch` now upcasts box corners to `float64` before computing areas and intersections, returning `float32`. This fixes integer-dtype overflow (e.g. `int32` coordinates around `50_000` could previously wrap to a negative area and produce an incorrect `0.0` IoU) and gives full `float64` precision to callers that pass `float64`/`int64` coordinates directly. It does not recover precision already lost when coordinates are stored as `float32` before this function is called (e.g. `Detections.xyxy`, which is `float32` throughout the library) — such callers must upcast their own arrays to `float64`/`int64` before calling `box_iou_batch` to benefit from this fix. Results for small-coordinate inputs are unchanged.