fix(core): add diff size limits to prevent UI freeze with large changesets

[UR-xiaoyang]

Issue for this PR

Closes #31916
Related to #35401, #33195, #29873

Type of change

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

What does this PR do?

This PR fixes the UI freeze/hang issue when opening the Review/Diff panel with large changesets (10,000+ files or large patches).

Root Cause:
The diff viewer was rendering unbounded diffs, causing the event loop to block when processing large changesets. This manifested as:

• TUI hanging at 100% CPU
• Web UI becoming completely unresponsive
• Process requiring force-kill

Solution:
Added configurable size limits with intelligent truncation:

1. Core changes (packages/core):

   • Added ConfigDiff.Info schema with max_files (default: 1000) and max_patch_bytes (default: 100KB)
   • Created truncateDiffs utility function with truncation logic
   • Added unit tests covering all truncation scenarios

2. TUI changes (packages/tui):

   • Applied truncation before rendering in diff viewer
   • Display truncation warning in header when limits exceeded

3. Web App changes (packages/app):

   • Applied truncation before rendering in review panel v2
   • Display truncation warnings in stats area

Users can customize limits via config:

{
  "diff": {
    "max_files": 2000,
    "max_patch_bytes": 204800
  }
}

How did you verify your code works?

1. Unit tests: Created comprehensive tests for truncateDiffs function covering:

   • File truncation when exceeding max_files
   • Patch truncation when exceeding max_patch_bytes
   • Default limits behavior
   • Diffs without patches
   • Combined file and patch truncation

2. Integration testing: Verified with standalone test simulating:

   • 15,000 files scenario (truncated to 1,000) ✓
   • 200KB patch scenario (truncated with notice) ✓
   • Normal workflow (50 files, no truncation) ✓
   • Custom config (respects user limits) ✓

All tests passed successfully.

Screenshots / recordings

N/A - This is a performance/stability fix without visual UI changes (only adds warning text when truncation occurs).

Checklist

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

[github-actions]

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

Related PRs Found

I found two potentially related PRs addressing similar issues:

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

   • Related to: UI freezing with diffs rendering
   • Overlap: Both address the same root cause (diff rendering performance), but take different approaches

2. PR fix: add large diff guard to TimelineDiffView to prevent render freeze #33198 - fix: add large diff guard to TimelineDiffView to prevent render freeze

   • Related to: Large diff rendering causing UI freeze
   • Overlap: Similar solution scope (guarding against large diffs), focuses on a specific component

These PRs address the same performance issue (UI freeze with large changesets) that PR #35546 is fixing. You may want to review if there's any overlap in implementation or if these previous attempts should be considered in the current solution.