CS-Cart thumbnail warmer (test stand) | 80 source images x 3 sizes = 240 thumbnails
================================================================
STEP 1. Purge thumbnail cache (what admin 'Clear cache' does)
        removed 0 files
STEP 2. COLD: generate every thumbnail (lazy path visitors hit)
        240 thumbnails in 6.22s => 25.9 ms per thumbnail
        => first view of a 24-card category page pays ~0.62s of image work alone
STEP 3. WARM: same catalog again (files exist, nginx static path)
        240 thumbnails (1.3 MB) read in 0.003s => 0.01 ms per thumbnail
RESULT: cold lazy generation is ~1988x slower than serving a warmed cache.
        Pre-warm after every cache clear / size change, and visitors never pay it.