feat(0.0.6): vram_budget % dropdown (create-time cap → engine vram_budget_mb) + RETURN_TYPES image·mask·latent_preview migration

Pairs with engine 0.0.12 (5660bfff): 4-engine consolidation
(ideogram/qwen-layered export + runtime FBCache + ideogram VAE-decode
cache cleanup). vram_budget is a create-time knob (changing it rebuilds
the pipeline). fbcache/fbcache_uncond cond/uncond split already landed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>