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/doc.go

@@ -0,0 +1,17 @@
+// Package normalize provides an OPTIONAL normalization hook for feedkit pipelines.
+//
+// Motivation:
+// Many daemons have sources that:
+//  1. fetch raw upstream data (often JSON), and
+//  2. transform it into a domain's normalized payload format.
+//
+// Doing both steps inside Source.Poll works, but tends to make sources large and
+// encourages duplication (unit conversions, common mapping helpers, etc.).
+//
+// This package lets a source emit a "raw" event (e.g., Schema="raw.openweather.current.v1",
+// Payload=json.RawMessage), and then a normalization processor can convert it into a
+// normalized event (e.g., Schema="weather.observation.v1", Payload=WeatherObservation{}).
+//
+// Key property: normalization is optional.
+// If no registered Normalizer matches an event, it passes through unchanged.
+package normalize