fix(ui-map): cut the visibility fog with an inverse stencil mask (Safari pan perf, stage 2)

Stage 1 (render-on-demand) removed the idle / whole-system freeze, but
panning a loaded map with "visible hyperspace" on stayed heavy in Safari:
the fog still cut its visibility holes by opaque overpaint — on KNNTS041
that is ~260 near-world-sized opaque circles blended over the fog every
rendered frame, a fill-rate cliff for Safari's WebGPU / Apple's tile-based
GPU.

Replace the overpaint with an INVERSE stencil mask: setVisibilityFog now
draws the FOG_COLOR rectangle(s) into fogLayer and collects the visibility
circles into one Graphics set as fogLayer.setMask({ mask, inverse: true }),
so the fog shows everywhere except the union of the circles. Per-frame cost
drops from dozens of blended opaque circle fills to one rect fill + a
stencil pass (no colour writes), which Apple's TBDR GPU handles cheaply,
and the fog stays fully vector — crisp at any zoom.

fogPaintOps and its unit tests are unchanged (the circle ops now feed the
mask instead of an overpaint). Verified with a high-contrast screenshot
during development (fog field with a correct circle-union hole) plus the
existing fog / render-on-demand e2e green on chromium + webkit.

Docs: renderer.md fog section + PLAN.md Phase 29 decision 9.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/PLAN.md

@@ -3293,9 +3293,26 @@ Decisions:
    is removed so a released drag stops instantly and the viewport
    goes idle immediately. `RendererHandle.getRenderCount()` (mirrored
    on `__galaxyDebug` as `getMapRenderCount`) backs the e2e
-   assertions. If Safari pan is still heavy after this, stage 2 cuts
-   the overpaint itself (an inverse stencil mask of the circle union,
-   kept vector so the map stays crisp at any zoom).
+   assertions. The owner confirmed this removed the idle / whole-system
+   freeze, but panning a loaded map with the fog on stayed heavy in
+   Safari (the overpaint fill-rate was untouched) — addressed in
+   decision 9.
+9. **Inverse stencil mask for the fog (fog perf, stage 2).** The fog's
+   visibility holes were previously cut by opaque background-coloured
+   circle overpaint — on a large report dozens of near-world-sized
+   opaque circles repainted every frame, the fill-rate cliff that kept
+   Safari's WebGPU pan heavy after stage 1. Stage 2 replaces the
+   overpaint with an INVERSE stencil mask: `setVisibilityFog` draws the
+   `FOG_COLOR` rectangle(s) into `fogLayer` and collects the visibility
+   circles into one `Graphics` set as
+   `fogLayer.setMask({ mask, inverse: true })`, so the fog shows
+   everywhere except the union of the circles. Per-frame cost drops to
+   one rectangle fill plus a stencil pass (no blended colour writes,
+   friendly to Apple's tile-based GPU), and the fog stays fully vector
+   — crisp at any zoom. `fogPaintOps` and its unit tests are unchanged
+   (the circle ops now feed the mask instead of an overpaint); the
+   rendered result is verified by a high-contrast screenshot during
+   development plus the existing fog / render-on-demand e2e.
 
 ## Phase 30. Calculator Tab