fix(ui): prevent highlight cache identity collisions

[benvinegar]

Summary

• replace sampled patch fingerprints with a memoized SHA-256 identity over the complete patch and parsed diff metadata
• include the file language in highlighted-diff cache identity
• add an adversarial regression proving equal-length patches that collided under sampling cannot reuse stale highlighted text
• add a patch changeset

Validation

• bun run typecheck
• bun run test — 1,591 pass, 2 skip
• bun run test:integration — 132 pass, 1 skip
• bun run test:tty-smoke — 9 pass
• bun run deps:check
• focused format and lint checks
• real-TTY source-diff smoke
• bun run benchmarks/highlight-cache-layers.ts
  • cold: 379.99 ms
  • main cache hit: 0.03 ms
  • worker cache revisit: 4.29 ms

This PR description was generated by Pi using gpt-5.6-sol

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
hunk-web	Ignored		Aug 27, 2026 7:46pm

[greptile-apps]

Greptile Summary

This PR replaces sampled highlight-cache fingerprints with memoized SHA-256 identities covering complete patch and parsed metadata content.

• Includes file language in highlighted-diff cache keys.
• Adds an adversarial regression test for formerly colliding equal-length patches.
• Adds a patch changeset documenting the stale-highlight fix.

Confidence Score: 5/5

The PR appears safe to merge, with the cache-identity fix covered by a targeted adversarial regression test.

The new key hashes all patch and metadata content, separates languages, preserves source-provider identity, and memoizes repeat hashing without exposing a concrete stale-cache, startup, or compatibility failure.

Important Files Changed

Filename	Overview
src/ui/diff/useHighlightedDiff.ts	Replaces collision-prone sampled identities with complete content hashing, memoizes fingerprints by immutable file snapshot, and includes language in the cache key.
src/ui/diff/useHighlightedDiff.test.ts	Adds a focused regression fixture proving patches that collide under the former sampling scheme receive distinct keys and highlighted output.
.changeset/fuzzy-diffs-refresh.md	Correctly records the user-visible cache-collision fix as a patch release.

_{Reviews (1): Last reviewed commit: "fix(ui): prevent highlight cache identit..." | Re-trigger Greptile}