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/normalizers/doc.go

@@ -36,6 +36,10 @@
 //  2. Provider-level shared helpers live under the provider directory:
 //     internal/normalizers/<provider>/
 //
+//     Use this for provider-specific quirks that should be shared by BOTH sources
+//     and normalizers (time parsing, URL/unit invariants, ID normalization, etc.).
+//     Keep these helpers pure (no I/O) and easy to test.
+//
 //     You may use multiple helper files (recommended) when it improves clarity:
 //     - types.go        (provider JSON structs)
 //     - common.go       (provider-shared helpers)