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/processor.go

@@ -0,0 +1,57 @@
+package normalize
+
+import (
+	"context"
+	"fmt"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+// Processor applies ordered normalization rules to pipeline events.
+//
+// Selection rule:
+//   - iterate in Normalizers order
+//   - the first Normalizer whose Match returns true is applied
+//
+// If no normalizer matches, the default behavior is pass-through.
+type Processor struct {
+	Normalizers []Normalizer
+
+	// If true, events that do not match any normalizer cause an error.
+	// Default is false (pass-through).
+	RequireMatch bool
+}
+
+// NewProcessor constructs a normalization processor from an ordered normalizer list.
+func NewProcessor(normalizers []Normalizer, requireMatch bool) Processor {
+	return Processor{
+		Normalizers:  append([]Normalizer(nil), normalizers...),
+		RequireMatch: requireMatch,
+	}
+}
+
+// Process implements processors.Processor.
+func (p Processor) Process(ctx context.Context, in event.Event) (*event.Event, error) {
+	for _, n := range p.Normalizers {
+		if n == nil {
+			continue
+		}
+		if !n.Match(in) {
+			continue
+		}
+
+		out, err := n.Normalize(ctx, in)
+		if err != nil {
+			return nil, fmt.Errorf("normalize: normalizer failed: %w", err)
+		}
+		return out, nil
+	}
+
+	if p.RequireMatch {
+		return nil, fmt.Errorf("normalize: no normalizer matched event (id=%s kind=%s source=%s schema=%q)",
+			in.ID, in.Kind, in.Source, in.Schema)
+	}
+
+	out := in
+	return &out, nil
+}