perf(weather): apply the infrared cloud key in the shell's shader

On the 3D Tiles host each Satellite clouds image (4096x2048 for the
regional product and for every detail window) was drawn to a canvas, read
back, given its transparency curve on the page and written back. The
lookup table halved that pass; this removes it. The shell's material now
samples the image as it was decoded and applies the same curve (brightest
channel, 2.2 power, smoothstep 0.40-0.70) in its fragment shader, scaled
by a strength uniform.

The shell's image cache no longer keys on the infrared mode, so changing
the filter strength sets one uniform: no fetch, no decode and no second
33 MB canvas per mode. Globe hosts keep the per-pixel transfer for their
draped imagery.

Co-Authored-By: Claude Fable 5.2 <[email protected]>

docs/CURRENT-STATE.md

@@ -891,9 +891,11 @@ image proxy accepts `bbox=west,south,east,north` for every product: inside the
 product bounds, 2:1 within 1 %, rounded to 0.25°, up to 4096×2048 (the
 default), cached by product, time, size and bbox. Diagnostics report
 `shell.detail` (`bbox`, `size`, `ready`, `enabled`).
-Clouds only applies a soft brightness ramp to decoded pixels once, using
+Clouds only applies a soft brightness ramp, using
 Cesium's sRGB-to-linear conversion (`channel ** 2.2`) and smoothstep from 0.40
-to 0.70. The old 0.55 threshold is the ramp midpoint. RGB and source alpha are
+to 0.70. Globe hosts apply it to decoded pixels once; a shell applies the same
+ramp in its fragment shader from a strength uniform, so its images are drawn
+as decoded and a strength change neither fetches nor rewrites an image. The old 0.55 threshold is the ramp midpoint. RGB and source alpha are
 preserved in Full image mode; both modes use the chosen layer opacity. This
 is a display filter, not a cloud mask. The cloud-filter slider is a continuous
 0–100% strength: 0% is Full, 100% is the existing Clouds only key, and
@@ -909,13 +911,13 @@ and processes it before staging. Globe hosts crop that canvas into 256 px tiles
 to maximum level 3 on a geographic 2×1 root grid bounded to the manifest extent,
 avoiding request-dependent contrast seams.
 On globe hosts, regional infrared retains network tiles and processes each
-decoded tile once; shells process each whole frame once.
+decoded tile once; shells never read a frame's pixels back.
 Exact-time tile and image responses are immutable for 24 hours; manifests and
 errors remain uncached. Each globe renderer retains up to 6 processed global
 mosaics in a least-recently-used cache keyed by observation time and infrared
 mode (up to 48 MiB of canvas pixels). Each shell keeps decoded full-extent and
 detail images in one least-recently-used cache bounded at 128 MiB, keyed by
-time, mode and window: the shown frame and the warmed next frame, each
+time and window: the shown frame and the warmed next frame, each
 full-extent and detail; nothing older survives them. Disable clears the caches.
 Cache hits skip
 fetch/decode and report `mosaic.cached: true` with zero decode time.