Multi-class beta polish: label summary wording + preview docs

[gbeane]

Summary

Two beta-readiness polish items for multi-class mode (SHOULD-DOs from the multi-class plan), so the preview ships with clear, non-misleading UX and documentation.

T12 — Multi-class-aware Label Summary

Previously the Label Summary always showed "Behavior" / "Not Behavior". In multi-class mode "Not Behavior" is misleading and typically stayed at 0, because explicit negatives are stored on the reserved None track rather than as NOT_BEHAVIOR on a single behavior.

• FrameLabelCountWidget row headers are now retitlable via set_class_labels(positive, negative).
• In multi-class mode the summary shows the selected behavior name and None; binary mode is unchanged ("Behavior" / "Not Behavior").
• Counts are sourced correctly per mode: the negative row now reflects the None-track BEHAVIOR counts in multi-class mode (cached as self._none_counts, refreshed for the current video like self._counts), and the selected behavior's NOT_BEHAVIOR counts in binary mode.

Known-limitations documentation

• New user-guide page Multi-Class Classification (Preview) (multi-class.md) describing the mode, how to enable it, labeling/None semantics, and known limitations (no post-processing, project-level training settings, mutual-exclusivity requirement, no migration, format may change).
• Added in both doc trees (docs/user-guide/ and the in-app src/jabs/resources/docs/user_guide/), registered in the in-app help tree, and cross-linked from classifier-types.md (distinguishing classifier type from classifier mode).

Tests

• New tests/ui/test_label_count_widget.py: default headers, set_class_labels retitles both frame/bout rows, and restore-to-default.
• Full tests/ui suite passes (69); ruff clean.

Notes

Binary mode behavior and wording are unchanged. Part of the multi-class beta-readiness work alongside #385/#386/#387 (section 10 fixes) and #388 (preview label).

[Copilot]

Pull request overview

Polishes multi-class (preview) UX by making the Label Summary wording and negative-class counts multi-class-aware, and adds a dedicated user-guide page documenting multi-class semantics and known limitations.

Changes:

• Add retitlable row headers to the Label Summary widget and wire it through the main control widget.
• Update label/bout count aggregation so multi-class negative counts come from the reserved None track (instead of NOT_BEHAVIOR on the selected behavior), and retitle rows accordingly.
• Add “Multi-Class Classification (Preview)” documentation to both the published docs and in-app help, and cross-link it from classifier type docs.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/ui/test_label_count_widget.py	Adds UI tests validating default and retitled Label Summary headers.
src/jabs/ui/main_window/central_widget.py	Implements multi-class-aware label summary retitling and None-track negative count sourcing/caching.
src/jabs/ui/main_control_widget/main_control_widget.py	Exposes a control-layer method to retitle Label Summary class labels.
src/jabs/ui/main_control_widget/label_count_widget.py	Stores row-header QLabel instances and adds set_class_labels() for retitling.
src/jabs/ui/dialogs/user_guide_dialog.py	Registers the new multi-class help page in the in-app user guide tree.
src/jabs/resources/docs/user_guide/multi-class.md	Adds in-app user guide page for multi-class preview mode.
src/jabs/resources/docs/user_guide/classifier-types.md	Cross-links classifier “type vs mode” and points to the multi-class page.
docs/user-guide/multi-class.md	Adds published docs page for multi-class preview mode.
docs/user-guide/classifier-types.md	Cross-links classifier “type vs mode” and points to the multi-class page.

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