feat(feedkit): add optional normalization hook and document external API

Introduce an optional normalization stage for feedkit pipelines via the new
normalize package. This adds:

- normalize.Normalizer interface with flexible Match() semantics
- normalize.Registry for ordered normalizer selection (first match wins)
- normalize.Processor adapter implementing pipeline.Processor
- Pass-through behavior when no normalizer matches (normalization is optional)
- Func helper for ergonomic normalizer definitions

Update root doc.go to fully document the normalization model, its role in the
pipeline, recommended conventions (Schema-based matching, raw vs normalized
events), and concrete wiring examples. The documentation now serves as a
complete external-facing API specification for downstream daemons such as
weatherfeeder.

This change preserves feedkit’s non-framework philosophy while enabling a
clean separation between data collection and domain normalization.

normalize/normalize.go

@@ -0,0 +1,76 @@
+package normalize
+
+import (
+	"context"
+	"fmt"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+// Normalizer converts one event shape into another.
+//
+// A Normalizer is typically domain-owned code (weatherfeeder/newsfeeder/...)
+// that knows how to interpret a specific upstream payload and produce a
+// normalized payload.
+//
+// Normalizers are selected via Match(). The matching strategy is intentionally
+// flexible: implementations may match on Schema, Kind, Source, or any other
+// Event fields.
+type Normalizer interface {
+	// Match reports whether this normalizer applies to the given event.
+	//
+	// Common patterns:
+	//   - match on e.Schema (recommended for versioning)
+	//   - match on e.Source (useful if Schema is empty)
+	//   - match on (e.Kind + e.Source), etc.
+	Match(e event.Event) bool
+
+	// Normalize transforms the incoming event into a new (or modified) event.
+	//
+	// Return values:
+	//   - (out, nil) where out != nil: emit the normalized event
+	//   - (nil, nil): drop the event (treat as policy drop)
+	//   - (nil, err): fail the pipeline
+	//
+	// Note: If you simply want to pass the event through unchanged, return &in.
+	Normalize(ctx context.Context, in event.Event) (*event.Event, error)
+}
+
+// Func is an ergonomic adapter that lets you define a Normalizer with functions.
+//
+// Example:
+//
+//	n := normalize.Func{
+//	  MatchFn: func(e event.Event) bool { return e.Schema == "raw.openweather.current.v1" },
+//	  NormalizeFn: func(ctx context.Context, in event.Event) (*event.Event, error) {
+//	    // ... map in.Payload -> normalized payload ...
+//	  },
+//	}
+type Func struct {
+	MatchFn     func(e event.Event) bool
+	NormalizeFn func(ctx context.Context, in event.Event) (*event.Event, error)
+
+	// Optional: helps produce nicer panic/error messages if something goes wrong.
+	Name string
+}
+
+func (f Func) Match(e event.Event) bool {
+	if f.MatchFn == nil {
+		return false
+	}
+	return f.MatchFn(e)
+}
+
+func (f Func) Normalize(ctx context.Context, in event.Event) (*event.Event, error) {
+	if f.NormalizeFn == nil {
+		return nil, fmt.Errorf("normalize.Func(%s): NormalizeFn is nil", f.safeName())
+	}
+	return f.NormalizeFn(ctx, in)
+}
+
+func (f Func) safeName() string {
+	if f.Name == "" {
+		return "<unnamed>"
+	}
+	return f.Name
+}