feat(core): add readiness adapters for map and visualization libraries (#1548)

Add readiness-only runtime adapters for Mapbox GL JS, Leaflet, Google
Maps, MapLibre GL JS, and D3. Each adapter gates `__renderReady` until
the library's async initialization completes, preventing the renderer
from capturing blank or half-loaded frames.

Built on the `getReadyPromise` adapter contract from #1543. A shared
`createReadinessAdapter()` helper in `_readiness.ts` owns the
settled-tracking WeakSet, promise-identity stability, and
`Promise.allSettled` gate — each adapter provides only its type, window
global name, and `waitFor` callback.

Readiness signals per library:
- Mapbox / MapLibre: `map.loaded()` + `map.on('load', ...)`
- Leaflet: `map.whenReady(cb)`
- Google Maps: `map.addListener('tilesloaded', cb)` with handle cleanup
- D3: `transition.end()` promise

50 unit tests across 5 test files covering happy path, no-instances,
stable promise identity, post-settle drain, loaded-before-subscribe
race, and listener cleanup. 5 producer regression tests with
Docker-generated baselines for end-to-end render verification.

packages/producer/tests/leaflet-adapter/meta.json

@@ -0,0 +1,13 @@
+{
+  "name": "leaflet-adapter",
+  "description": "Regression guard for the Leaflet runtime adapter. Verifies that a real Leaflet map with OSM tiles initializes and registers on window.__hfLeaflet for deterministic frame capture.",
+  "tags": ["regression", "adapter"],
+  "minPsnr": 25,
+  "maxFrameFailures": 10,
+  "minAudioCorrelation": 0,
+  "maxAudioLagWindows": 1,
+  "renderConfig": {
+    "fps": 30,
+    "workers": 1
+  }
+}