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/doc.go

@@ -0,0 +1,22 @@
+// Package processors defines feedkit's generic processor abstraction and registry.
+//
+// Processors are optional pipeline stages that can transform, drop, or reject
+// events before dispatch to sinks.
+//
+// Registry provides name-based construction so daemons can assemble processor
+// chains without embedding switch statements in wiring code.
+//
+// Example:
+//
+//	reg := processors.NewRegistry()
+//	reg.Register("normalize", func() (processors.Processor, error) {
+//	    return normalize.NewProcessor(myNormalizers, false), nil
+//	})
+//
+//	chain, err := reg.BuildChain([]string{"normalize"})
+//	if err != nil {
+//	    // handle wiring error
+//	}
+//
+//	p := &pipeline.Pipeline{Processors: chain}
+package processors