Replaces 'serious' as the default boot animation. Serious is still available via ?boot=serious. Spring mode draws a field of procedural turtle-graphics flowers (parametric petals, stems, leaves, pollen) over a cream/butter gradient, with the title and MOTD characters each animating independently with per-char font swaps, hue shifts, rotation, and bob/jitter. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>