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(...)`

processors/processor.go

@@ -0,0 +1,15 @@
+package processors
+
+import (
+	"context"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+// 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)
+}
+
+// Factory constructs a configured Processor instance.
+type Factory func() (Processor, error)