docs: add differential diffusion seam fix design

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-02-25-differential-diffusion-seam-fix-design.md

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+# Differential Diffusion Seam Fix
+
+## Problem
+
+The current seam fix pass uses binary masks (1.0/0.0) with `SetLatentNoiseMask`. This creates hard transitions at band edges that can themselves become visible artifacts. Differential diffusion allows gradient masks where the value controls per-pixel denoise intensity, producing smoother seam repairs.
+
+## Design
+
+### GenerateSeamMask Node Changes
+
+Add a `mode` combo input:
+
+- **`binary`** (default): Current behavior. Output is 1.0 inside seam bands, 0.0 outside.
+- **`gradient`**: Linear falloff from 1.0 at seam center to 0.0 at band edge. Value at distance `d` from center: `max(0, 1.0 - d / half_w)`. Where horizontal and vertical bands overlap (grid intersections), take `max` of both values.
+
+The `seam_width` parameter keeps the same meaning in both modes.
+
+### Workflow Changes
+
+Add one `DifferentialDiffusion` node (node 24) inside the Seam Fix group. It wraps the model before it reaches the seam fix KSampler:
+
+- Checkpoint → DifferentialDiffusion → Seam Fix KSampler (replaces direct Checkpoint → KSampler link)
+- All other wiring unchanged. `SetLatentNoiseMask` still passes the mask to the latent.
+
+### Tests
+
+- Existing binary tests pass with explicit `mode="binary"`
+- Gradient tests: center=1.0, edge=0.0, midpoint~0.5, intersection uses max