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_test.go

@@ -0,0 +1,139 @@
+package normalize
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+	"time"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+func TestProcessorFirstMatchWins(t *testing.T) {
+	var firstCalls, secondCalls int
+
+	p := NewProcessor([]Normalizer{
+		Func{
+			MatchFn: func(event.Event) bool { return true },
+			NormalizeFn: func(_ context.Context, in event.Event) (*event.Event, error) {
+				firstCalls++
+				out := in
+				out.Schema = "normalized.first.v1"
+				return &out, nil
+			},
+		},
+		Func{
+			MatchFn: func(event.Event) bool { return true },
+			NormalizeFn: func(_ context.Context, in event.Event) (*event.Event, error) {
+				secondCalls++
+				out := in
+				out.Schema = "normalized.second.v1"
+				return &out, nil
+			},
+		},
+	}, false)
+
+	out, err := p.Process(context.Background(), testEvent())
+	if err != nil {
+		t.Fatalf("Process error: %v", err)
+	}
+	if out == nil {
+		t.Fatalf("expected output event, got nil")
+	}
+	if out.Schema != "normalized.first.v1" {
+		t.Fatalf("unexpected schema: %q", out.Schema)
+	}
+	if firstCalls != 1 {
+		t.Fatalf("expected first normalizer called once, got %d", firstCalls)
+	}
+	if secondCalls != 0 {
+		t.Fatalf("expected second normalizer skipped, got %d calls", secondCalls)
+	}
+}
+
+func TestProcessorNoMatchPassThroughAndRequireMatch(t *testing.T) {
+	in := testEvent()
+	in.Schema = "raw.schema.v1"
+
+	passThrough := NewProcessor([]Normalizer{
+		Func{
+			MatchFn: func(event.Event) bool { return false },
+			NormalizeFn: func(_ context.Context, in event.Event) (*event.Event, error) {
+				out := in
+				out.Schema = "should.not.run"
+				return &out, nil
+			},
+		},
+	}, false)
+
+	out, err := passThrough.Process(context.Background(), in)
+	if err != nil {
+		t.Fatalf("pass-through Process error: %v", err)
+	}
+	if out == nil {
+		t.Fatalf("expected pass-through output event, got nil")
+	}
+	if out.Schema != "raw.schema.v1" {
+		t.Fatalf("expected unchanged schema, got %q", out.Schema)
+	}
+
+	required := NewProcessor(nil, true)
+	_, err = required.Process(context.Background(), in)
+	if err == nil {
+		t.Fatalf("expected require-match error")
+	}
+	if !strings.Contains(err.Error(), "no normalizer matched") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestProcessorDropAndErrorPropagation(t *testing.T) {
+	t.Run("drop", func(t *testing.T) {
+		p := NewProcessor([]Normalizer{
+			Func{
+				MatchFn: func(event.Event) bool { return true },
+				NormalizeFn: func(context.Context, event.Event) (*event.Event, error) {
+					return nil, nil
+				},
+			},
+		}, false)
+
+		out, err := p.Process(context.Background(), testEvent())
+		if err != nil {
+			t.Fatalf("Process error: %v", err)
+		}
+		if out != nil {
+			t.Fatalf("expected nil output for dropped event, got %#v", out)
+		}
+	})
+
+	t.Run("error", func(t *testing.T) {
+		p := NewProcessor([]Normalizer{
+			Func{
+				MatchFn: func(event.Event) bool { return true },
+				NormalizeFn: func(context.Context, event.Event) (*event.Event, error) {
+					return nil, errors.New("map failed")
+				},
+			},
+		}, false)
+
+		_, err := p.Process(context.Background(), testEvent())
+		if err == nil {
+			t.Fatalf("expected error")
+		}
+		if !strings.Contains(err.Error(), "normalizer failed") {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+}
+
+func testEvent() event.Event {
+	return event.Event{
+		ID:        "evt-normalize-1",
+		Kind:      event.Kind("observation"),
+		Source:    "source-1",
+		EmittedAt: time.Now().UTC(),
+		Payload:   map[string]any{"x": 1},
+	}
+}