Comment Coverage Pass

Goal

Add a narrow, high-value non-React pass for @platejs/comment around the only worthwhile seams:

• BaseCommentPlugin.ts
• getCommentKeys.ts
• getCommentCount.ts

Constraints

• Fast-lane only.
• No /react.
• Cover withComment through the plugin override, not a fake standalone unit test.
• Stop after the helper seam and the real plugin API/transform seam.

Slice

1. Add tiny helper specs for comment key extraction and comment counting.
2. Add one BaseCommentPlugin contract spec for:
   • document-wide node / nodes search with at: []
   • nodeId behavior for normal and draft leaves
   • setDraft
   • unsetMark for overlapping and single comments
   • normalize cleanup of stray comment: true
3. Allow one runtime fix only if the direct tests expose an actual bug.

Notes

• There were no existing package specs.
• node() and nodes() default to the current selection, so document-wide tests need at: [].