Fix: cannot add point label after selecting a keypoint

[C-Achard]

This PR fixes keypoint switching and out-of-order annotation when Napari events occur during an add.
Closes #233

Fix

• Update next point properties inside napari's block_update_properties() so selected points are not relabeled and remain selected
• Captures the requested (bodypart, individual) pair once at the start of add(), such that duplicate detection, property assignment, and QUICK-mode lookup use a consistent keypoint
• Resolves the backing layer once per operation without rebinding it

Why

Previously, add() read current_keypoint multiple times. Updating layer data could trigger events that changed the current properties mid-operation, causing the new point to receive an incorrect pair or leaving the store in an inconsistent sequence state.

Tests

Regression tests added:

• Switching the complete (bodypart, individual) pair does not relabel or deselect an existing point
• Changing only the individual preserves the bodypart and existing annotations
• Switching to an already annotated keypoint does not modify point data or properties
• Changing the current keypoint does not affect another independently managed Points layer

[Copilot]

Pull request overview

Fixes keypoint switching while points are selected and stabilizes annotation properties during point addition.

Changes:

• Updates current properties without relabeling selected points.
• Snapshots the requested keypoint during annotation.
• Adds regression tests for selection and layer isolation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/napari_deeplabcut/core/keypoints.py	Revises keypoint property updates and add behavior.
src/napari_deeplabcut/_tests/core/test_keypoints.py	Adds keypoint-switching regression tests.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/napari_deeplabcut/core/keypoints.py:330

• prev_keypoint() still calls self._keypoints.index(self.current_keypoint) without handling ValueError. Since next_keypoint() now explicitly handles the case where current_keypoint is outside the configured sequence, prev_keypoint() can still crash in the same scenario (e.g., if events or malformed current_properties produce a keypoint not present in self._keypoints). Consider mirroring the defensive behavior here as well.

    def prev_keypoint(self, *args):
        ind = self._keypoints.index(self.current_keypoint) - 1
        if ind >= 0:
            self.current_keypoint = self._keypoints[ind]

[deruyter92]

Good fix!