sources: standardize Event.ID on Source:EffectiveAt; simplify raw event helper

- Adopt an opinionated Event.ID policy across sources:
  - use upstream-provided ID when available
  - otherwise derive a stable ID from Source:EffectiveAt (RFC3339Nano, UTC)
  - fall back to Source:EmittedAt when EffectiveAt is unavailable
- Add common/id helper to centralize ID selection logic and keep sources consistent
- Simplify common event construction by collapsing SingleRawEventAt/SingleRawEvent
  into a single explicit SingleRawEvent helper (emittedAt passed in)
- Update NWS/Open-Meteo/OpenWeather observation sources to:
  - compute EffectiveAt first
  - generate IDs via the shared helper
  - build envelopes via the unified SingleRawEvent helper
- Improve determinism and dedupe-friendliness without changing schemas or payloads

internal/sources/nws/observation.go

@@ -4,7 +4,6 @@ package nws
 import (
 	"context"
 	"encoding/json"
-	"fmt"
 	"strings"
 	"time"
 
@@ -15,10 +14,6 @@ import (
 )
 
 // ObservationSource polls an NWS station observation endpoint and emits a RAW observation Event.
-//
-// This corresponds to URLs like:
-//
-//	https://api.weather.gov/stations/KSTL/observations/latest
 type ObservationSource struct {
 	http *common.HTTPSource
 }
@@ -36,35 +31,14 @@ func NewObservationSource(cfg config.SourceConfig) (*ObservationSource, error) {
 
 func (s *ObservationSource) Name() string { return s.http.Name }
 
-// Kind is used for routing/policy.
-// We keep Kind canonical (observation) even for raw events; Schema differentiates raw vs canonical.
 func (s *ObservationSource) Kind() event.Kind { return event.Kind("observation") }
 
-// Poll fetches NWS "latest observation" and emits exactly one RAW Event.
-// The RAW payload is json.RawMessage and Schema is standards.SchemaRawNWSObservationV1.
 func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 	raw, meta, err := s.fetchRaw(ctx)
 	if err != nil {
 		return nil, err
 	}
 
-	// Event.ID must be set BEFORE normalization (feedkit requires it).
-	// Prefer NWS-provided "id" (stable URL). Fallback to a stable computed key.
-	eventID := strings.TrimSpace(meta.ID)
-	if eventID == "" {
-		ts := meta.ParsedTimestamp
-		if ts.IsZero() {
-			ts = time.Now().UTC()
-		}
-
-		station := strings.TrimSpace(meta.StationID)
-		if station == "" {
-			station = "UNKNOWN"
-		}
-
-		eventID = fmt.Sprintf("nws:observation:%s:%s:%s", s.http.Name, station, ts.UTC().Format(time.RFC3339Nano))
-	}
-
 	// EffectiveAt is optional; for observations it’s naturally the observation timestamp.
 	var effectiveAt *time.Time
 	if !meta.ParsedTimestamp.IsZero() {
@@ -72,11 +46,15 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 		effectiveAt = &t
 	}
 
+	emittedAt := time.Now().UTC()
+	eventID := common.ChooseEventID(meta.ID, s.http.Name, effectiveAt, emittedAt)
+
 	return common.SingleRawEvent(
 		s.Kind(),
 		s.http.Name,
 		standards.SchemaRawNWSObservationV1,
 		eventID,
+		emittedAt,
 		effectiveAt,
 		raw,
 	)
@@ -84,18 +62,13 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 
 // ---- RAW fetch + minimal metadata decode ----
 
-// observationMeta is a *minimal* decode of the NWS payload used only to build
-// a stable Event.ID and a useful EffectiveAt for the envelope.
 type observationMeta struct {
 	ID         string `json:"id"`
 	Properties struct {
-		StationID string `json:"stationId"`
 		Timestamp string `json:"timestamp"`
 	} `json:"properties"`
 
-	// Convenience fields populated after decode.
 	ParsedTimestamp time.Time `json:"-"`
-	StationID       string    `json:"-"`
 }
 
 func (s *ObservationSource) fetchRaw(ctx context.Context) (json.RawMessage, observationMeta, error) {
@@ -106,12 +79,10 @@ func (s *ObservationSource) fetchRaw(ctx context.Context) (json.RawMessage, obse
 
 	var meta observationMeta
 	if err := json.Unmarshal(raw, &meta); err != nil {
-		// If metadata decode fails, still return raw; envelope will fall back to computed ID.
+		// If metadata decode fails, still return raw; envelope will fall back to Source:EffectiveAt.
 		return raw, observationMeta{}, nil
 	}
 
-	meta.StationID = strings.TrimSpace(meta.Properties.StationID)
-
 	tsStr := strings.TrimSpace(meta.Properties.Timestamp)
 	if tsStr != "" {
 		if t, err := time.Parse(time.RFC3339, tsStr); err == nil {