Fix partial-word boundaries in inline completions

[seeARMS]

Fixes #1.

Accepting a completion after The quick brow could save The quick brow n fox.... Ask the model for a completed sentence beginning with the exact existing prefix, then remove that prefix without trimming the insertion boundary. Partial words, normal word separators, existing spaces, apostrophes, and punctuation keep their correct placement. Rewritten or missing prefixes are ignored.

Validation:

• Reproduced the original bug in Chromium before the fix: Tab saved The quick brow n fox jumps over the lazy dog.
• All 11 browser regression tests passed against the production build. These control only the model response and drive the real editor, ghost text, Tab/Escape handlers, and saved-draft reload. Includes consecutive spaces, contractions, punctuation, and word-limit clipping.
• Drove Chrome with its real Gemini Nano: typed The quick brow, accepted the ghost text with Tab, and confirmed The quick brown fox jumps over the lazy dog. persisted after reload.
• npm run typecheck, npm run build, and git diff --check passed.

Adds reproducible Playwright tests and runs them in CI. Model wording remains nondeterministic; the parser requires the existing prefix to remain unchanged before showing a suggestion.

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Status	Name	Latest Commit	Updated (UTC)
🔵 In progress View logs	draftside	3132f4c	Sep 06 2026, 04:54 PM