fix: surrogate pair corruption in structural char protection fallback backspace/delete

[xiaolai]

Closes #695

Summary

• Use findClusterBreak from @codemirror/state instead of naive head ± 1 in the non-structural cursor fallback path of smartBackspace and smartDelete
• The old code split surrogate pairs (emoji, astral-plane CJK) when a multi-cursor session had at least one cursor at a structural position (table pipe, list marker, etc.)
• Added regression tests with emoji surrogate pairs for both backspace and delete paths

Test plan

• New tests: surrogate pair backspace + delete with multi-cursor structural protection
• All 663 test files pass (pnpm check:all)
• Build succeeds

🤖 Generated with Claude Code

[xiaolai]

✅ VERIFIED — The fix replaces the naive head - 1 / head + 1 fallbacks with CodeMirror's findClusterBreak(), which correctly handles surrogate pairs and grapheme clusters, and includes regression tests for both backspace and delete with emoji at non-structural cursors.