Harden input editor: grapheme-safe selection, cursor boundaries, and paste

[RtlZeroMemory]

Summary

• Hardened applyInputEditEvent for grapheme-safe cursoring and editing, including word-wise movement (Ctrl+Left/Right) and stricter boundary normalization.
• Added single-line text selection support for input editing:
  • Shift+Left/Right/Home/End
  • Shift+Ctrl+Left/Right
  • Ctrl+A
  • non-shift movement collapse behavior
  • type/backspace/delete/paste replacement of active selections (forward and backward ranges)
• Wired selection state through WidgetRenderer so focused input instances persist normalized selection alongside cursor state.
• Added deterministic runtime test coverage for unicode behavior, cursor boundaries, paste handling, selection flows, and state invariants.
• Updated input/focus docs to document unicode behavior, movement rules, paste semantics, and renderer-facing selection state.

Tests

• npm run build
• npm run typecheck
• npm run lint
• node scripts/run-tests.mjs

All commands pass locally.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Make Ctrl+Left word movement linear-time

prevWordBoundary repeatedly calls prevBoundary inside loops, but prevBoundary scans from offset 0 to position each time, so a single Ctrl+Left (and Shift+Ctrl+Left) over long word runs becomes quadratic in input length. On large pasted inputs this can cause noticeable key-handling lag because each word-step recomputes earlier boundaries from scratch instead of reusing a single pass.

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