refactor(providers): centralize provider-specific parsing and invariants

- Introduce internal/providers/nws with shared timestamp parsing used by both
  NWS sources and normalizers
- Migrate NWS observation source + normalizer to use the shared provider helper
  for consistent RFC3339/RFC3339Nano handling
- Introduce internal/providers/openweather with a shared URL invariant helper
  enforcing units=metric
- Remove duplicated OpenWeather URL validation logic from the observation source
- Align provider layering: move provider “contract/quirk” logic out of
  normalizers and into internal/providers
- Update normalizer and standards documentation to clearly distinguish:
  provider helpers (internal/providers) vs canonical mapping logic
  (internal/normalizers)

This refactor reduces duplication between sources and normalizers, clarifies
layering boundaries, and establishes a scalable pattern for future forecast
and alert implementations.

internal/standards/doc.go

@@ -10,5 +10,6 @@
 // provider-specific logic and free of dependencies on internal/sources/* or
 // internal/normalizers/* to avoid import cycles.
 //
-// Provider-specific decoding and mapping lives in internal/normalizers/<provider>.
+// Provider-specific decoding helpers and quirks live in internal/providers/<provider>.
+// Normalizer implementations and canonical mapping logic live in internal/normalizers/<provider>.
 package standards