refactor: move event system to reducer, remove engine package

Move the event emitter and lifecycle events from the engine into the reducer,
making the reducer the single point of orchestration for reduction.
This eliminates the engine package entirely.

- Add events.go to pkg/reducer with Start, Step, and Stop events.
- Extend Reducer interface to embed Emitter and add Expression() method.
- Update NormalOrderReducer to embed BaseEmitter and emit lifecycle events.
- Update all plugins to attach to Reducer instead of Engine.
- Remove internal/engine package.
- Add Off() method to BaseEmitter to complete Emitter interface.
- Fix Emitter.On signature to use generic type E instead of string.

pkg/reducer/events.go

@@ -0,0 +1,13 @@
+package reducer
+
+// Event represents lifecycle events during reduction.
+type Event int
+
+const (
+	// StartEvent is emitted before reduction begins.
+	StartEvent Event = iota
+	// StepEvent is emitted after each reduction step.
+	StepEvent
+	// StopEvent is emitted after reduction completes.
+	StopEvent
+)

pkg/reducer/reducer.go

@@ -2,14 +2,26 @@
 // reduction strategies.
 package reducer
 
-import "git.maximhutz.com/max/lambda/pkg/expr"
+import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+	"git.maximhutz.com/max/lambda/pkg/expr"
+)
 
 // Reducer defines the interface for expression reduction strategies.
 // Different evaluation modes (normal order, applicative order, SKI combinators,
 // etc.) implement this interface with their own reduction logic.
+//
+// Reducers also implement the Emitter interface to allow plugins to observe
+// reduction lifecycle events (Start, Step, Stop).
 type Reducer interface {
-	// Reduce performs all reduction steps on the expression, calling onStep
-	// after each reduction.
+	emitter.Emitter[Event]
+
+	// Reduce performs all reduction steps on the expression.
+	// Emits StartEvent before reduction, StepEvent after each step, and
+	// StopEvent after completion.
 	// Returns the final reduced expression.
-	Reduce(e expr.Expression, onStep func(expr.Expression)) expr.Expression
+	Reduce(e expr.Expression) expr.Expression
+
+	// Expression returns the current expression state.
+	Expression() expr.Expression
 }