perf(wind): decode forecast GRIB in a worker and reuse the cycle's wind

Each /api/wind refresh decoded two or three 0.25 degree GRIB2 messages with
ecCodes (WASM), validated ~1.04 M doubles per message and resampled them, all
synchronously on the server thread. Measured with cached upstream bytes: 43
to 164 ms of event-loop block per warm refresh, 230 to 381 ms for the first
one in a process, 316 to 974 ms when all six keys refresh together.

Decode, validation and resampling now run on one lazily started worker
thread, which transfers back only the display-grid samples. The server keeps
the sampled U/V of the last forecast object per model (1.04 MB each), so an
overlay change, or a refresh within the same forecast step, fetches and
decodes only the scalar. Manifests and grid bodies are byte-identical to
before for the same upstream bytes.

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

server/providers/wind/ifs.js

@@ -1,6 +1,6 @@
-import { fetchText, fetchRange, loadOptionalScalar } from './gfs.js';
-import { decodeWindGribMessage } from './decode.js';
-import { resampleWeatherSnapshot, weatherScalarMetadata } from './grid.js';
+import { fetchText, loadWeatherSnapshot } from './gfs.js';
+import { decodeWindGribInWorker } from './decoder.js';
+import { weatherScalarMetadata } from './grid.js';
 
 /** Keyless ECMWF Open Data root for real-time IFS forecasts. */
 const IFS_BASE = 'https://data.ecmwf.int/forecasts';
@@ -110,9 +110,10 @@ export async function fetchIfsWind({
   fetchImpl = fetch,
   now = () => Date.now(),
   targetDx = 1,
-  decodeImpl = decodeWindGribMessage,
+  decodeImpl = decodeWindGribInWorker,
   signal,
   overlay = 'none',
+  windCache,
 } = {}) {
   weatherScalarMetadata(overlay);
   const nowMs = now();
@@ -137,24 +138,18 @@ export async function fetchIfsWind({
       /* A missing optional field does not discard valid wind. */
     }
   }
-  // All selected messages come from this one issue/valid-time object.
-  const load = async (range) =>
-    decodeImpl(
-      await fetchRange({ url: urls.grib, ...range, fetchImpl, signal }),
-    );
-  const [u, v, scalar] = await Promise.all([
-    load(ranges.u),
-    load(ranges.v),
-    loadOptionalScalar({
-      range: scalarRange,
-      url: urls.grib,
-      fetchImpl,
-      decodeImpl,
-      signal,
-    }),
-  ]);
-  signal?.throwIfAborted();
-  const fields = resampleWeatherSnapshot({ u, v, scalar, overlay, targetDx });
+  const fields = await loadWeatherSnapshot({
+    model: 'ifs',
+    url: urls.grib,
+    ranges,
+    scalarRange,
+    overlay,
+    targetDx,
+    fetchImpl,
+    decodeImpl,
+    signal,
+    windCache,
+  });
   const runIso = new Date(runMs).toISOString();
   const validIso = new Date(runMs + forecastHour * 3600_000).toISOString();
   return {