fix: merge overlapping multi-cursor selections after Shift+Arrow

[xiaolai]

Closes #692

Summary

• Pass merge=true to normalizeRangesWithPrimary() when extend=true in both horizontalMovement.ts and inputHandling.ts (vertical arrow handler)
• Overlapping selections from Shift+Arrow are now merged immediately, matching VS Code/Sublime Text behavior
• Prevents incorrect text insertion/deletion when typing with overlapping ranges

Test plan

• New tests: overlapping selections merge after Shift+Right extend
• New tests: adjacent selections merge when they grow to overlap
• New tests: non-overlapping selections remain separate
• New tests: end-to-end Shift+Arrow then type produces correct text
• All 17,758 existing tests pass
• Lint clean, build passes

🤖 Generated with Claude Code

[xiaolai]

✅ VERIFIED — The fix passes extend (true when Shift is held) as the merge parameter to normalizeRangesWithPrimary() in both horizontalMovement.ts:117 and inputHandling.ts:284, which is exactly option (a) from the audit finding — overlapping selections are now merged immediately after Shift+Arrow movement, preventing stale-position edits.