fix(#429/#408): bump _STAGING_FORMAT_VERSION 2->3 to invalidate poisoned SVDQ staging cache

049c5dc gated vae_comfyui_alias on _is_comfyui_flat_vae but did NOT bump the
staging format version, so machines with a cached SVDQ staging dir carrying the
stale 'vae_comfyui_alias: true' reuse it as-is -> the engine remaps the VAE's
post_quant_conv.bias request to conv2.bias -> 'Tensor post_quant_conv.bias not
found' on qwenimage-svdq (Qwen-Image-2512, a complete diffusers VAE). Bumping
the version changes every staging id so poisoned dirs are no longer reused and
the corrected alias-free config is regenerated.