[codex] Improve NB2 masked edit guidance

[yha9806]

Summary

• Updates the Gemini/NB2 masked edit adapter to render alpha masks as visible black/white reference masks before sending them to Gemini.
• Tightens adapter prompt wording so the visible mask is treated as coordinate guidance, not output content.
• Keeps the formal redraw API contract unchanged: callers still provide source crop + RGBA mask via inpaint_with_mask.

Validation

• /opt/homebrew/bin/python3.11 -m pytest tests/test_mask_refine.py tests/test_layers_redraw.py tests/test_layers_redraw_crop_pipeline.py tests/test_layers_redraw_mask_aware.py tests/test_layers_redraw_quality_gates.py tests/test_layers_redraw_refinement.py tests/test_layers_redraw_strategy.py tests/test_layers_redraw_wire_contract.py tests/test_layers_v2_redraw.py tests/test_mcp_layers_redraw_advisory.py tests/test_redraw_review_contract.py tests/test_redraw_showcase_review.py tests/test_gemini_image_size.py tests/test_provider_edit_capabilities.py -> 143 passed, 5 warnings
• git diff --check -> passed
• Secret scan over changed files for Gemini/OpenAI key-shaped tokens -> no matches
• Real NB2 probe on the roadside yellow flower crop reached formal redraw path and improved from gate-to-empty to constrained non-empty output: route_chosen=inpaint, refinement_applied=true, provider_requires_mask_for_edits=true, raw_scene_rejected=false, output alpha 0.0249% of canvas, bbox (2627, 1484, 2700, 1549).

Visual Risk

This is a provider-adapter quality improvement, not a full visual finish. The latest NB2 output no longer contaminates broad grass/vehicle regions and is localized to the flower footprint, but it can still render a chunky yellow patch because Gemini is not a native alpha-mask edit endpoint. Further work should tune scale/alpha extraction or use a provider with native masked edits for final showcase evidence.

Stacking

Stacked on PR #42 (codex/v0-23-redraw-replacement-contract). Keep this draft until #42 lands or retarget after merge.