feat: add profile diff command

[kacper-mikolajczak]

Summary

• Adds profile diff <before.json> <after.json> CLI command to compare two profiling exports
• No daemon required - operates purely on the exported JSON files from profile export
• Matches components by displayName, computes avg duration and render count deltas
• Categorizes into: regressed, improved, new, removed (5% threshold filters noise)

Builds on #38 (profile export).

Usage

# Capture baseline
agent-react-devtools profile start
# ... interact ...
agent-react-devtools profile stop
agent-react-devtools profile export before.json

# Make code changes, restart app

# Capture after
agent-react-devtools profile start
# ... same interaction ...
agent-react-devtools profile stop
agent-react-devtools profile export after.json

# Compare
agent-react-devtools profile diff before.json after.json

Example output

Before: 5 commits, 234.5ms total
After:  5 commits, 189.2ms total
Delta:  -45.3ms (-19%)

Regressed (1):
  Sidebar                        avg:   2.1ms ->    4.8ms   +129%  renders: 5 -> 12

Improved (2):
  ExpensiveList                  avg:  12.3ms ->    4.1ms    -67%  renders: 47 -> 12
  TodoItem                       avg:   2.1ms ->    0.8ms    -62%  renders: 94 -> 94

New (1):
  MemoizedHeader                 avg:       - ->    0.3ms           renders: - -> 8

Test plan

• 8 new unit tests: extractStats aggregation, regressed/improved/new/removed detection, threshold filtering, summary totals
• All 84 tests pass
• Manual: export two profiles, run diff, verify output

[changeset-bot]

🦋 Changeset detected

Latest commit: cf0977a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
agent-react-devtools	Minor

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[piotrski]

@kacper-mikolajczak, I've merged #38 with some fixes so the export works correctly with React DevTools (flame graphs, durations, etc.). Could you rebase this branch on the latest main? Let me know if you run into any conflicts.

[kacper-mikolajczak]

@piotrski Rebased - looks like we are good to go ✅

[piotrski]

Hey @kacper-mikolajczak, I've merged #38 with some fixes so the export works correctly with React DevTools (flame graphs, durations, etc.). Could you rebase this branch on the latest main? Let me know if you run into any conflicts.

[kacper-mikolajczak]

Sure, I will check that in the latest version 👍