perf(coding-agent): fast-path single-line intra-line diffs

[code-yeongyu]

Summary

Ports WI-5 from plans/port-gajae-3day-remaining-optimizations.md.

• adds conservative fast paths for single-line intra-line diff rendering
• falls back to Diff.diffWords unless the replacement span is provably byte-equivalent
• adds parity/no-diffWords regression tests and a word-diff benchmark suite

Evidence

RED:

• cd packages/coding-agent && npx tsx ../../node_modules/vitest/dist/cli.js --run test/diff-intraline-fastpath.test.ts test/diff-intraline-no-diffwords.test.ts
• Failed before implementation: 2 files, 4 failed tests.

GREEN:

• Same targeted vitest command passed: 2 files, 4 tests.
• Reran by worker, reviewer, and root verification.

Manual/data-surface QA:

• node scripts/run-pr530-benchmarks.mjs --suite word-diff --iterations 30 --json /tmp/wi5-word-diff-final-head.json --baseline /tmp/wi5-word-diff-base.json --expect-median-improvement 10 --allow-regression-pct 0
• Median: 22.34375ms -> 8.114834ms (63.68% improvement)
• Comparison failures: []

Adversarial review:

• First reviewer found a parity bug for punctuation-adjacent replacements, e.g. " a-" -> " --".
• Added the regression and tightened the fast path to ASCII word-character replacement spans.
• Second reviewer returned confirmed / UNCONDITIONAL APPROVAL.

Full check:

• npm run check passed locally and in the pre-commit hook.
• It still prints existing Biome info diagnostics outside this scoped change.

Cleanup

• No staged files remain after commit.
• /tmp/wi5-punctuation-probe.ts removed.
• No ulw-qa/WI-5 tmux session, browser context, server, or container was left running.

Plan: plans/port-gajae-3day-remaining-optimizations.md

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Summary by cubic

Adds a conservative fast path for single-line intra-line diffs in the coding agent to speed up rendering while keeping output identical. Implements WI-5 with ~64% median improvement in the word-diff suite.

• New Features

  • Fast path for identical lines and single-span ASCII word replacements (incl. long lines); otherwise falls back to Diff.diffWords.
  • Added parity and no-diffWords tests, plus a word-diff benchmark and baseline wired into scripts/run-pr530-benchmarks.mjs.

• Bug Fixes

  • Fixed punctuation-adjacent replacement highlighting by limiting the fast path to ASCII word tokens and adding a regression test.

^{Written for commit 2e9ee8b. Summary will update on new commits.}