Fetch independent weather sources concurrently

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