feat(extmarks): add clickable handlers for text buffers

[tobwen]

Summary

Adds a new clickable text-extmark feature for text buffers. Closes #809

This enables downstream interactions such as expand/edit/collapse summaries, preserves click targeting from mousedown, ignores drag-selection for click dispatch, and keeps typeId indexing correct during updates.

Verification

• added targeted tests for typeId update handling 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

[simonklee]

This pull request does not safely implement clickable text marks. Normal clicks
do not run their handlers, and clicks can run a handler for different text. It
also conflicts with the current main branch. Keeping #809 open.