Add profiling script

[trotzig]

Summary

Adds profile.js — a script to benchmark imageDiff across all snapshot pairs. Run it with:

node profile.js

Baseline numbers (3 runs each, macOS)

Snapshot	Min	Median	Mean	Max	Image sizes
long-example	2030ms	2038ms	2051ms	2085ms	1080×5485 / 1080×5678
tall-content-change-and-shift	867ms	869ms	873ms	883ms	1536×5000 / 1536×5000
dark-narrow-content-change-and-shift	214ms	237ms	235ms	253ms	390×5000 / 390×5000
different-widths	174ms	174ms	177ms	185ms	1280×720 / 1270×1811
blank-graph	114ms	127ms	143ms	188ms	1504×891 / 1504×891
large-tiny-text-change	86ms	86ms	86ms	87ms	1536×975 / 1536×975
removed-bottom-content	35ms	35ms	35ms	35ms	1908×234 / 1908×80
dark-text-change	17ms	17ms	17ms	17ms	686×428 / 686×428
removed-top-content	4ms	5ms	5ms	5ms	200×240 / 200×200
logo	1ms	2ms	2ms	2ms	64×64 / 80×80
Total (sum of means)			3624ms

Tall images dominate — long-example alone accounts for ~57% of total time, consistent with the LCS algorithm being O(n²) in image height.

🤖 Generated with Claude Code

[trotzig]

I haven't reviewed this deeply.