feat(air-quality): add the Air quality layer to the Weather group

Reworks #568 to the shape asked for in review.

Generalized from "AQHI" to a provider-backed Air quality layer, with ECCC's
AQHI as the first registered provider. Readings are carried as
{ provider, scale, value, band } rather than a bare number, because
air-quality indices are not interchangeable: AQHI runs 1-10+ on health risk,
the US AQI runs 0-500 on concentration breakpoints, and Europe's EAQI is a
five-step category. Naming the index a value belongs to is what lets another
network be added without inventing a shared numeric meaning. Coverage is
stated as "Canada only" on the provider, the layer row and the card.

Adds the WEATHER panel readout (readout: true) carrying the band legend and
the worst readings currently in view, with getRowControls/setRowControlsListener
following the wind and cyclone cards. An unresolvable view rectangle degrades
to the worst readings overall rather than to a blank card, and a rectangle
crossing the antimeridian is handled.

update() now takes the earthquakes layer's abort-and-guard shape: it aborts any
in-flight request, passes the signal through to the source, and refuses to
touch the scene when the request was superseded, aborted, or the layer was
disabled while it was outstanding. disable() and destroy() abort too. Tests
cover the late-response, superseded-refresh and disabled-update paths.

Rebased on main: the source is registered in src/sources/reference.js, and the
allocation profile was re-measured against the current worker — median 3,900
B/frame, max 4,114, 85.7 B/candidate, inside the shared 154 ceiling, budgeted
at 5,200 for 26% headroom. The earlier figures no longer held.

Freshness is now judged on the observation time rather than on when the last
response arrived. lastUpdate only says the feed answered; a station that keeps
returning a reading from four hours ago would otherwise be narrated as live by
the HUD and the voice provenance envelope. getStats() reports observedAt and
stale, so layerFeedState() calls the layer stale past two missed hourly
publications, and degraded when a refresh fails over readings that are still
good.

The share-link token is 0, which is what layer-token:next allocates. Taking it
moves the ledger fixtures that had hard-coded 0 as the free digit and 3 as the
one after; those shift to 3 and 4. The pinned equality between the reservation
ledger and LEGACY_LAYER_STATE_TOKENS also had to become "the closed legacy set
plus air-quality", since that set is documented as never growing. Happy to
re-pin this differently if you would rather the fixtures derive the provisional
digits from nextLayerStateToken() so the next layer does not move them again.

The layer and its source are registered as ./layers/air-quality and
./layers/air-quality/source, each with its own boundary group, following the
perimeters pair. check:boundaries requires the package exports and the group
exports to be the same set, so a layer that ships a source needs both.

src/layers/airQuality/source.js

@@ -0,0 +1,78 @@
+import { normalizeAqhiStations, normalizeAqhiObservations } from './model.js';
+
+const COLLECTIONS_ROOT = 'https://api.weather.gc.ca/collections';
+const STATIONS_URL = `${COLLECTIONS_ROOT}/aqhi-stations/items`;
+const OBSERVATIONS_URL = `${COLLECTIONS_ROOT}/aqhi-observations-realtime/items`;
+
+/** Comfortably above Canada's ~134 stations without inviting a huge page. */
+const STATION_LIMIT = 500;
+const OBSERVATION_LIMIT = 500;
+/** Station geometry is near-static — a few changes a year. Refetch twice a day. */
+const STATION_TTL_MS = 12 * 60 * 60 * 1000;
+
+/**
+ * Request and validate a complete ECCC AQHI snapshot before it can replace
+ * displayed readings. The first registered air-quality provider.
+ *
+ * Two collections are involved. `aqhi-stations` is the catalog — it supplies
+ * every reading its coordinates and is cached for twelve hours because it
+ * barely changes. `aqhi-observations-realtime` is the hourly reading feed,
+ * requested with `latest=true` so the response is current values rather than
+ * the full observation history, which is roughly 2,000 rows nationally.
+ *
+ * The station catalog is only replaced when a refresh actually returns
+ * stations, so a transient empty response cannot blank the layer.
+ *
+ * @param {object} [options]
+ * @param {Function} [options.fetchImpl] - Injected fetch, for tests.
+ * @param {() => number} [options.now] - Clock seam, for tests.
+ * @returns {{getSnapshot: Function}}
+ */
+export function createEcccAirQualitySource({
+  fetchImpl = (...args) => globalThis.fetch(...args),
+  now = () => Date.now(),
+} = {}) {
+  /** @type {Map<string, object>} */
+  let stations = new Map();
+  let stationsFetchedAt = 0;
+
+  const getJson = async (url, signal) => {
+    signal?.throwIfAborted();
+    const response = await fetchImpl(url, {
+      headers: { Accept: 'application/json' },
+      signal,
+    });
+    if (!response.ok) throw new Error(`ECCC HTTP ${response.status}`);
+    const payload = await response.json();
+    signal?.throwIfAborted();
+    return payload;
+  };
+
+  return {
+    async getSnapshot({ signal } = {}) {
+      const clock = now();
+      if (stations.size === 0 || clock - stationsFetchedAt > STATION_TTL_MS) {
+        const payload = await getJson(
+          `${STATIONS_URL}?f=json&limit=${STATION_LIMIT}`,
+          signal,
+        );
+        const next = normalizeAqhiStations(payload);
+        if (!next) throw new Error('Malformed AQHI station response');
+        // An empty refresh must not wipe a good catalog.
+        if (next.size > 0) {
+          stations = next;
+          stationsFetchedAt = clock;
+        }
+      }
+      if (stations.size === 0) throw new Error('No AQHI stations available');
+
+      const payload = await getJson(
+        `${OBSERVATIONS_URL}?f=json&latest=true&limit=${OBSERVATION_LIMIT}`,
+        signal,
+      );
+      const readings = normalizeAqhiObservations(payload, stations, clock);
+      if (!readings) throw new Error('Malformed AQHI observation response');
+      return readings;
+    },
+  };
+}