Add an optional MASK output driven by a mask_cells string param (row-major, 1-indexed cell list). The interactive picker gains a mask toggle: click cells to mark which denoise (amber) vs stay preserved (green). Stitch helpers now also return per-cell boxes so the mask aligns to the grid in both auto and manual modes. Enables masked/partial-denoise grid workflows (SetLatentNoiseMask). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>