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/providers/openweather/doc.go

@@ -0,0 +1,8 @@
+// Package openweather contains provider-specific helper code for OpenWeather used by
+// both sources and normalizers.
+//
+// Rules:
+//   - No network I/O here.
+//   - Keep helpers deterministic and easy to unit test.
+//   - Put provider invariants here (e.g., units=metric enforcement).
+package openweather