fix(tui): eliminate O(n) mention-trigger scan on every keystroke - #49634

Issue for this PR

Closes #49635

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Typing or dictating a large amount of text into the TUI chat input made it lag badly as the buffer grew, because mentionTriggerIndex runs on every keystroke (onContentChange → autocomplete onInput) and was doing two full-buffer Intl.Segmenter passes per keystroke — measured ~65ms at 100KB of input.

The fix adds two cheap raw-string gates so the display-aware scan only runs when an actual @ trigger needs resolving:

1. no @ in the buffer → return immediately (~0.001ms)
2. cursor at the end of an ASCII buffer → resolve the trailing whitespace-delimited word with a plain scan instead (~0.04ms)

Non-ASCII text, mid-buffer cursors and active @ mentions still take the exact original code path, so behavior is unchanged. A randomized parity test (~500 cases) additionally proves the new fast paths return identical results to the old implementation.

How did you verify your code works?

• bun test in packages/tui: 202 pass, 0 fail (includes the new test/util/display.test.ts)
• tsgo --noEmit: clean
• oxlint: 0 errors

Screenshots / recordings

N/A – no visual change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR