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touch the scene when the request was superseded, aborted, or the layer was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cover the late-response, superseded-refresh and disabled-update paths.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.

DATA_SOURCES.md

@@ -31,6 +31,7 @@ How to read this:
 | **The Space Devs — Launch Library 2 v2.3**                            | Recent launch, payload, stage, and recovery metadata for Space Missions (30d)                                                       | [The Space Devs terms of use](https://github.com/TheSpaceDevs/Tutorials/blob/main/faqs/faq_TSD.md#terms-of-use): data may be used and shared in any form; avoid forwarding it without added value; attribution is encouraged (not mandatory). [Official API limits](https://ll.thespacedevs.com/docs/): 15 unauthenticated calls/hour; optional token | "Launch Library 2 — The Space Devs" (courtesy attribution)                                                                                  |
 | **Esri World Imagery** (ArcGIS Online tile service)                   | The keyless satellite basemap — the default landing when no Google/ion credential is configured, and the "Esri Satellite" map stack | [Esri Master Agreement](https://www.esri.com/en-us/legal/terms/full-master-agreement): the public World Imagery service is usable in public-facing apps with attribution; no key is required for this classic endpoint, but Esri governs and can change access — an app at scale should review current ArcGIS Location Platform terms                 | "Powered by Esri — Source: Esri, Maxar, Earthstar Geographics, and the GIS User Community" (provider carries the service's own credit line) |
 | **USGS**                                                              | Earthquakes                                                                                                                         | U.S. public domain                                                                                                                                                                                                                                                                                                                                    | "Data courtesy of the U.S. Geological Survey"                                                                                               |
+| **Environment and Climate Change Canada** (MSC GeoMet OGC API) | Air quality (AQHI) — Canadian station observations | [ECCC Data Servers End-use Licence](https://eccc-msc.github.io/open-data/licence/readme_en/) (open, attribution required, no endorsement implied) | "Contains information licensed under the Environment and Climate Change Canada Data Servers End-use Licence" |
 | **NASA GIBS** (WMTS tiles + Worldview Snapshots) and **NASA CMR** (granule catalog) | Recent Imagery layer: HLS S30/L30 (Sentinel-2, Landsat 8/9) and VIIRS true-colour tiles draped on the map, per-day thumbnails and PNG exports; the HLS granule catalog (dates, cloud, footprints) for a selected box | U.S. public domain (NASA data); keyless, browser-direct (CORS `*`); no bulk use — one 30-day catalog query per box, tiles bounded to the box, six tile requests in flight | "We acknowledge the use of imagery provided by services from NASA's Global Imagery Browse Services (GIBS), part of NASA's Earth Science Data and Information System (ESDIS)." — [gibs.earthdata.nasa.gov](https://gibs.earthdata.nasa.gov), [HLS](https://lpdaac.usgs.gov/products/hlss30v002/) |
 | **NIFC WFIGS** (Wildland Fire Interagency Geospatial Services, ArcGIS feature service) | Fire Perimeters layer: current interagency wildfire incident perimeters, containment, and incident facts | U.S. public domain (interagency wildland-fire data published through the [NIFC Open Data portal](https://data-nifc.opendata.arcgis.com/)); fetched keyless through `/api/fire-perimeters` (5-minute server cache) with `exceededTransferLimit` paging | "Wildfire perimeters: National Interagency Fire Center (WFIGS)" |
 | **InciWeb** (inciweb.wildfire.gov)                                    | Per-incident "InciWeb ↗" card links to official incident pages (narratives, evacuation/closure notices) | U.S. government public incident information. Links are matched best-effort by incident name and state, with incident page origin and update times checked on selection. Same-origin `/api/fire-perimeters/inciweb` routes cache the catalog for 1 hour and publication metadata for 30 minutes; linked pages remain InciWeb content | "Incident information: InciWeb (inciweb.wildfire.gov)" |
@@ -72,6 +73,14 @@ The optional Local ADS-B layer and the broadcast-FM receiver use radio you recei
 
 ### Notes on the live sources
 
+- **Air quality (ECCC AQHI).** Keyless. Read from the MSC GeoMet OGC API (`api.weather.gc.ca`): the `aqhi-stations` catalog plus `aqhi-observations-realtime`. **Coverage is Canada only** — ECCC is the single registered provider, and the layer row and readout card both say so. Roughly 134 stations across ECCC's five administrative zones (Atlantic, Quebec, Ontario, Prairies & North, Pacific & Yukon). AQHI publishes hourly, so readings poll every 20 minutes and the station catalog, which changes a few times a year, is refetched only every 12 hours. Observations are requested with `latest=true`; without it the collection returns the full history, roughly 2,000 rows nationally. Readings older than six hours are dropped rather than displayed as current.
+
+  Readings are stored 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 later without inventing a shared numeric meaning that does not exist.
+
+  AQHI is reported by ECCC as an integer from 1 to 10 with an open-ended "10+" above that. The API returns the underlying decimal (1.08, 2.32); the layer rounds to the published form rather than displaying a precision the index does not claim.
+
+  **Quebec is thinly represented**, and that is upstream rather than a filter here: the province runs its own Info-Smog program, so only a handful of Quebec stations appear in ECCC's national feed. The reporting station count also varies hour to hour as stations report or fall silent.
+
 - **Google Maps Platform.** You supply your own API key and are bound by [Google's ToS](https://cloud.google.com/maps-platform/terms). Google Maps Content (tiles, geocodes, places) **may not be cached, stored, rehosted, or committed** — this app only ever uses it live, which is the compliant pattern. The "Google" attribution is displayed on the globe and must stay visible. Restrict your key (see [SECURITY.md](SECURITY.md)).
 - **OpenSky Network.** Its license is **non-commercial**, and operational use of the REST API in a live product can require a prior written agreement with OpenSky — even for non-profit/government use. If you deploy this commercially, contact OpenSky for your own terms. The flights layer is a toggle and runs anonymously by default.
 - **adsb.lol flight fallback.** When OpenSky is unavailable and no last-good OpenSky response exists, the server requests a cached, capped 250 nm adsb.lol point snapshot around the current camera subpoint. This is regional observed context, not worldwide completeness; provenance is exposed in the Flights stats/context row. Military ICAOs remain reconciled through the existing dedicated military registry rather than duplicated.