fix(ui): prepare diffs off the render thread

[jerrydong1988]

Issue for this PR

Closes #34437

Type of change

• Bug fix
• Refactor / code improvement

What does this PR do?

Moves expensive diff preparation off the UI render thread into a Web Worker to prevent UI freezes on large diffs (e.g., C++ projects like llama.cpp on Windows).

Root cause: execEditLength ran synchronously in the render thread during diff computation, blocking frame rendering for hundreds of milliseconds.

Changes:

• New diff infrastructure in packages/ui/src/diff/: worker, client, protocol, resource, client-core
• Updated session components (review, turn, message-part, apply-patch) to use createPreparedDiff instead of synchronous diff parsing
• Components now receive pending or prepared state and remain pure renderers

Performance (full-context replacement, 1,536 old + 1,536 new lines):

Metric	Before	After	Change
Max frame gap	233ms	67ms	71% lower
p95 frame gap	33ms	17ms	50% lower

Original implementation by @Hona. Rebased onto latest dev with updated imports for session-ui migration and inlined worker helpers to avoid circular dependencies.

How did you verify your code works?

• Original author ran 5 isolated Chromium runs showing 71% frame gap reduction
• bun test passes for diff infrastructure unit tests
• bun typecheck passes
• Built and used on Windows with large C++ diff sessions — no freeze
• GitHub Actions CI build + package succeeds
• render-purity.test.ts updated to exclude worker.ts (the worker IS the diff parser)

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Related PR Found

PR #31309: fix(ui): prepare diffs off the render thread
#31309

This is the original PR that the current PR (#34415) is rebasing. The PR description explicitly states: "Replaces PR #31309" and credits the original implementation to @Hona. PR #34415 is a rebase of #31309 onto the latest dev branch with updated import paths and resolved conflicts.

Other potentially related PRs addressing similar performance issues:

• PR fix: add large diff guard to TimelineDiffView to prevent render freeze #33198: fix: add large diff guard to TimelineDiffView to prevent render freeze – addresses render freeze with large diffs
• PR fix(app): avoid O(n^2) dedup hang on large diff summaries #30441: fix(app): avoid O(n^2) dedup hang on large diff summaries – related performance issue with diffs

However, since PR #34415 explicitly replaces/rebases PR #31309, that's the key relationship. PRs #33198 and #30441 are separate fixes for related (but different) performance issues.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

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