Fetch independent weather sources concurrently

docs/integrations/weatherapi.md

@@ -28,7 +28,10 @@ fetch before source requests begin.
 
 The adapter makes one source request for each endpoint, subject to retry on
 transient failures. A successful readiness request supplies the current
-conditions source response.
+conditions source response. The remaining independent source requests run
+concurrently, then their results are processed in the source order shown below.
+This keeps source provenance, missing-source policy, and surfaced errors
+deterministic regardless of response order.
 
 | Source | Endpoint | Query parameters | Availability |
 | --- | --- | --- | --- |

docs/internal/collect.md

@@ -15,6 +15,11 @@ The adapter's successful readiness request for current conditions is reused as
 that normalized source; collection does not trigger a second identical current
 conditions request.
 
+After readiness succeeds, the adapter obtains the other independent source
+responses concurrently. It merges their normalized results in the established
+source order, so provenance, warnings, and source-local failures remain
+deterministic. Cancellation remains authoritative for every in-flight request.
+
 The package wraps adapter construction failures, including an invalid Weather
 API base URL, as weather-collection setup errors and fetch failures as
 bundle-collection errors. It does not retry, persist, select reports, derive