refactor!: introduce generic processors registry and remove normalize registry adapter

- add new `processors` package with canonical `Processor` interface
- add `processors.Registry` with Register/Build/BuildChain factory model
- switch `pipeline.Pipeline` to `[]processors.Processor`
- replace `normalize.Registry` + registry adapter with direct `normalize.Processor`
- remove `normalize/registry.go`
- update root docs to position normalize as one optional processing stage
- add tests for processors registry, normalize processor behavior, and pipeline flow

BREAKING CHANGE:
- `pipeline.Processor` removed; use `processors.Processor`
- `normalize.Registry` and old normalize processor adapter APIs removed
- downstream daemons must update processor wiring to new `processors.Registry`
  and `normalize.NewProcessor(...)`

normalize/doc.go

@@ -1,4 +1,4 @@
-// Package normalize provides an OPTIONAL normalization hook for feedkit pipelines.
+// Package normalize provides a concrete normalization processor for feedkit pipelines.
 //
 // Motivation:
 // Many daemons have sources that:
@@ -9,9 +9,9 @@
 // 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
+// Payload=json.RawMessage), and then a normalize.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.
+// If no Normalizer matches an event, Processor passes it through unchanged by default.
 package normalize