results(gate-b): mask UX fix evidence — translucent overlay, 38ms knob response

- Updated RESULTS.md with mask UX fix section
- Screenshot shows translucent light green overlay, photo visible
- Playwright timing: sensitivity 38ms, edge offset 38ms, sheet switch 21ms
- Tip: 866de43 (fix + tests committed and deployed)

Fixes: issue #55

test-results/gate-b-2026-09-07-fix/RESULTS.md

@@ -43,3 +43,42 @@
 - File selection processes images and advances to mask step
 - All event handlers (buttons, selects, ranges) use Svelte 5 syntax
 - **Gate B not green** — remaining product-state issues (TV-3/6/7/10) are separate
+
+---
+
+## Mask UX Fix (issue #55) — 2026-09-07
+
+### Problem
+iPad @ https://192.168.1.202/: (1) mask preview nearly solid green — slab not visible;
+(2) Sensitivity/Edge/Feather sliders take forever to respond.
+
+### Root Causes
+1. **Overlay too opaque**: max alpha 140/255 (~55% green wash) made the photo hard to see.
+2. **Auto-sheet fallback wrong**: when border was ambiguous, defaulted to green; if no green
+   pixels existed, mask became all-slab (all 255) → entire canvas green.
+3. **No debounce**: `applyMaskKnobs` called full O(n²) remask on every `oninput` event
+   (every slider tick), blocking the UI thread.
+
+### Fix Applied
+- **Translucent overlay**: green from (0,200,0) → (60,220,60), max alpha 140 → 80 (~31% opacity)
+- **Auto-sheet fallback**: when ambiguous, try both green and black; pick whichever produces
+  non-trivial slab area (1%-99%). Falls back to green if both degenerate.
+- **Knob debounce**: oninput → cheap overlay redraw with previous mask (rAF-limited);
+  onchange (release) → immediate full remask; 150ms debounced remask during drag.
+- **Playwright E2E tests**: overlay visual, sensitivity knob 38ms, edge offset knob 38ms,
+  sheet switch 21ms.
+
+### Evidence
+- Screenshot: `frontend/test-results/mask-ux-evidence/mask-overlay-translucent.png`
+  (translucent light green overlay, photo visible underneath)
+- Playwright timing: sensitivity knob 38ms, edge offset 38ms, sheet switch 21ms — all <300ms
+
+### Deployment
+- Commits: `a1d4a96` (fix), `866de43` (Playwright tests)
+- Branch: `gate-b`
+- Pushed to: `origin/gate-b`
+- Redeployed on: ubuDual5060
+- Health: `GET https://192.168.1.202/api/health` → 200 `{"status":"ok"}`
+
+### Status
+- **Gate B not green** — mask UX is usable; remaining product-state issues are separate.