feat(nodes): t2i img2img via init_img socket + init_img_strength widget

QuantFuncImageList: add init_img (IMAGE) socket + init_img_strength
(FLOAT, default 0.6) widget for #293 t2i img2img (SDEdit). When init_img
is connected, QuantFuncGenerate builds the T2I pipeline (edit_mode=False,
enable_img2img=True) and routes strength → options_json edit_strength →
flow-match SDEdit init in generate(). main_image is now optional;
combine() requires exactly one of main_image (edit) / init_img (img2img),
init_img wins if both set.

Node layout: ref_image_2..10 grouped at top, mask_config + main_image_mask
grouped together, init_img + init_img_strength grouped at the bottom.

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