Slate v2 Op-family Twenty-first Slice

Supporting plan. For current queue and roadmap truth, see master-roadmap.md.

Goal

• Broaden collapsed delete(...) across mixed-inline sibling leaves inside one supported top-level block.

Scope

• Keep the slice collapsed-delete only.
• Support:
  • explicit Point
  • current collapsed selection
  • reverse
  • distance
• Support crossing sibling text leaves inside one supported top-level block, including inline descendants.
• Avoid arbitrary explicit multi-leaf Range deletion in this slice.

Phases

• Confirm the mixed-inline collapsed-delete semantics and current location seam
• Write focused failing tests
• Implement the smallest honest core/API slice
• Sync package/public docs
• Verify the touched package/docs