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/openmeteo/common.go

@@ -1,19 +0,0 @@
-// FILE: ./internal/normalizers/openmeteo/common.go
-package openmeteo
-
-import (
-	"time"
-
-	openmeteo "gitea.maximumdirect.net/ejr/weatherfeeder/internal/providers/openmeteo"
-)
-
-// parseOpenMeteoTime parses Open-Meteo timestamps.
-//
-// The actual parsing logic lives in internal/providers/openmeteo so both the
-// source (envelope EffectiveAt / event ID) and normalizer (canonical payload)
-// can share identical timestamp behavior.
-//
-// We keep this thin wrapper to avoid churn in the normalizer package.
-func parseOpenMeteoTime(s string, tz string, utcOffsetSeconds int) (time.Time, error) {
-	return openmeteo.ParseTime(s, tz, utcOffsetSeconds)
-}