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/registry_test.go

@@ -0,0 +1,100 @@
+package processors
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+type testProcessor struct {
+	name string
+}
+
+func (p testProcessor) Process(context.Context, event.Event) (*event.Event, error) {
+	return nil, nil
+}
+
+func TestRegistryRegisterValidation(t *testing.T) {
+	t.Run("empty driver panics", func(t *testing.T) {
+		r := NewRegistry()
+		assertPanics(t, func() {
+			r.Register("   ", func() (Processor, error) { return testProcessor{name: "x"}, nil })
+		})
+	})
+
+	t.Run("nil factory panics", func(t *testing.T) {
+		r := NewRegistry()
+		assertPanics(t, func() {
+			r.Register("normalize", nil)
+		})
+	})
+
+	t.Run("duplicate driver panics", func(t *testing.T) {
+		r := NewRegistry()
+		r.Register("normalize", func() (Processor, error) { return testProcessor{name: "a"}, nil })
+		assertPanics(t, func() {
+			r.Register("normalize", func() (Processor, error) { return testProcessor{name: "b"}, nil })
+		})
+	})
+}
+
+func TestRegistryBuildUnknownDriver(t *testing.T) {
+	r := NewRegistry()
+	_, err := r.Build("does_not_exist")
+	if err == nil {
+		t.Fatalf("expected error for unknown driver")
+	}
+	if !strings.Contains(err.Error(), "unknown processor driver") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestRegistryBuildChainPreservesOrder(t *testing.T) {
+	r := NewRegistry()
+	r.Register("first", func() (Processor, error) { return testProcessor{name: "first"}, nil })
+	r.Register("second", func() (Processor, error) { return testProcessor{name: "second"}, nil })
+
+	chain, err := r.BuildChain([]string{"first", "second"})
+	if err != nil {
+		t.Fatalf("BuildChain error: %v", err)
+	}
+	if len(chain) != 2 {
+		t.Fatalf("expected 2 processors, got %d", len(chain))
+	}
+
+	p0, ok := chain[0].(testProcessor)
+	if !ok || p0.name != "first" {
+		t.Fatalf("unexpected chain[0]: %#v", chain[0])
+	}
+	p1, ok := chain[1].(testProcessor)
+	if !ok || p1.name != "second" {
+		t.Fatalf("unexpected chain[1]: %#v", chain[1])
+	}
+}
+
+func TestRegistryBuildChainIndexedFailure(t *testing.T) {
+	r := NewRegistry()
+	r.Register("ok", func() (Processor, error) { return testProcessor{name: "ok"}, nil })
+	r.Register("broken", func() (Processor, error) { return nil, errors.New("boom") })
+
+	_, err := r.BuildChain([]string{"ok", "broken"})
+	if err == nil {
+		t.Fatalf("expected error")
+	}
+	if !strings.Contains(err.Error(), "chain[1]") {
+		t.Fatalf("expected indexed error, got: %v", err)
+	}
+}
+
+func assertPanics(t *testing.T, fn func()) {
+	t.Helper()
+	defer func() {
+		if recover() == nil {
+			t.Fatalf("expected panic")
+		}
+	}()
+	fn()
+}