feat([issue-912]): minimal-mutation regen (empty prompt) + large-image handling

[atomantic]

Summary

Follow-up tuning to the SynthID-defeat Regenerate action (#958), driven by interactive testing on the 128 GB machine.

• Minimal-mutation default — regen now runs with no prompt and no negative prompt: a near-pure VAE round-trip + a low denoise pass. This overwrites the per-pixel watermark with the least visible change to the image, instead of re-steering it toward the source's text description. (generateImage waives the prompt-required guard when regenOf is set; both the mflux and FLUX.2 runners accept an empty --prompt. An explicit promptOverride still allows a creative re-roll.)
• Lower denoise — default strength 0.4 → 0.25 (provisional) and the validation floor 0.2 → 0.1, so the minimum strength that defeats SynthID can be swept live via the API strength param and locked once known.
• Large-image handling — clampRegenDimensions caps the render to a ~2 MP FLUX budget (PORTOS_REGEN_MAX_MP, /16-rounded, aspect-preserved); a 12.6 MP (4096×3072) render would otherwise stall/OOM the O(tokens²) attention pass. generateImage then upscales the result back to the source's exact dimensions (Lanczos), so a 4K source returns a watermark-free copy at 4K. The sidecar records both delivered and render dimensions.

Refs #912

Test plan

• regen.test.js — clampRegenDimensions (under-budget passthrough, 12.6 MP downscale to ≤2 MP /16 aspect-preserved, non-/16 rounding, custom budget, garbage-dim fallback); buildRegenParams empty-prompt default, promptOverride, clamp + upscaleTo to exact source dims.
• Existing regen/local/route suites updated for the new defaults; 118 tests green in the three affected files.
• Verified both runners accept an empty --prompt (mflux is None check; FLUX.2 argparse required=True is satisfied by an empty value).
• Manual: confirmed live on this machine that a codex source regenerates through schnell img2img end-to-end (feat([issue-912]): regenerate gallery images to defeat SynthID watermarks #958); this PR's behavior change is exercised post-merge by tuning strength via the API against a SynthID detector.