[HAS PR] feat: clickable text extmarks for expand/collapse interactions

[tobwen]

Summary

Add first-class clickable text extmark support so downstream TUIs can implement features like expand/edit/collapse summaries.

Scope

• support click-capable extmark handlers in text buffers
• preserve click target via the original mousedown offset
• ignore drag-selection for click handler dispatch
• keep typeId reindexing correct when extmarks are updated

Verification

• added targeted tests for typeId update behavior and the drag-selection click regression
• bun run build:lib passes
• bun test src/lib/extmarks.test.ts src/renderables/__tests__/Textarea.selection.test.ts segfaults on local Bun 1.3.10

Screenshots / recordings

The closed state currently has no interactivity; video shows the final status.