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(...)`

pipeline/pipeline.go

@@ -5,15 +5,11 @@ import (
 	"fmt"
 
 	"gitea.maximumdirect.net/ejr/feedkit/event"
+	"gitea.maximumdirect.net/ejr/feedkit/processors"
 )
 
-// Processor can mutate/drop events (dedupe, rate-limit, normalization tweaks).
-type Processor interface {
-	Process(ctx context.Context, in event.Event) (out *event.Event, err error)
-}
-
 type Pipeline struct {
-	Processors []Processor
+	Processors []processors.Processor
 }
 
 func (p *Pipeline) Process(ctx context.Context, e event.Event) (*event.Event, error) {