actor: introduce generic Mailbox interface with iter.Seq support

diff --git a/actor/mailbox.go b/actor/mailbox.go
new file mode 100644
index 0000000..4d15f43
--- /dev/null
+++ b/actor/mailbox.go
@@ -0,0 +1,176 @@
+package actor
+
+import (
+	"context"
+	"iter"
+	"sync"
+	"sync/atomic"
+)
+
+// Mailbox represents the message queue for an actor. It provides methods for
+// sending messages and receiving them via an iterator pattern.
+type Mailbox[M Message, R any] interface {
+	// Send attempts to send an envelope to the mailbox with context-based
+	// cancellation. Returns true if sent successfully, false if the
+	// context was cancelled or the mailbox is closed.
+	Send(ctx context.Context, env envelope[M, R]) bool
+
+	// TrySend attempts to send without blocking. Returns true if the
+	// envelope was sent, false if the mailbox is full or closed.
+	TrySend(env envelope[M, R]) bool
+
+	// Receive returns an iterator for consuming messages from the mailbox.
+	// The iterator will yield messages until the mailbox is closed or the
+	// context is cancelled.
+	Receive(ctx context.Context) iter.Seq[envelope[M, R]]
+
+	// Close closes the mailbox, preventing new messages from being sent.
+	// Any remaining messages can still be consumed via Receive.
+	Close()
+
+	// IsClosed returns true if the mailbox has been closed.
+	IsClosed() bool
+
+	// Drain returns an iterator that yields all remaining messages in the
+	// mailbox after it has been closed. This is useful for cleanup.
+	Drain() iter.Seq[envelope[M, R]]
+}
+
+// ChannelMailbox is a channel-based implementation of the Mailbox interface.
+type ChannelMailbox[M Message, R any] struct {
+	ch     chan envelope[M, R]
+	closed atomic.Bool
+
+	// mu protects Send/TrySend operations to prevent send-on-closed-channel
+	// panics. Close() acquires write lock, Send/TrySend acquire read lock.
+	mu sync.RWMutex
+
+	// closeOnce ensures Close() executes exactly once.
+	closeOnce sync.Once
+
+	// actorCtx is the actor's context for lifecycle management.
+	actorCtx context.Context
+}
+
+// NewChannelMailbox creates a new channel-based mailbox with the specified
+// buffer capacity and actor context.
+func NewChannelMailbox[M Message, R any](actorCtx context.Context,
+	capacity int) *ChannelMailbox[M, R] {
+
+	if capacity <= 0 {
+		capacity = 1
+	}
+	return &ChannelMailbox[M, R]{
+		ch:       make(chan envelope[M, R], capacity),
+		actorCtx: actorCtx,
+	}
+}
+
+// Send implements Mailbox.Send with context-aware blocking send.
+func (m *ChannelMailbox[M, R]) Send(ctx context.Context,
+	env envelope[M, R]) bool {
+
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	if m.IsClosed() {
+		return false
+	}
+
+	select {
+	case m.ch <- env:
+		return true
+	case <-ctx.Done():
+		return false
+	case <-m.actorCtx.Done():
+		// Actor is shutting down.
+		return false
+	}
+}
+
+// TrySend implements Mailbox.TrySend with non-blocking send.
+func (m *ChannelMailbox[M, R]) TrySend(env envelope[M, R]) bool {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	if m.IsClosed() {
+		return false
+	}
+
+	select {
+	case m.ch <- env:
+		return true
+	default:
+		return false
+	}
+}
+
+// Receive implements Mailbox.Receive using iter.Seq pattern.
+func (m *ChannelMailbox[M, R]) Receive(
+	ctx context.Context) iter.Seq[envelope[M, R]] {
+	return func(yield func(envelope[M, R]) bool) {
+		for {
+			select {
+			case env, ok := <-m.ch:
+				if !ok {
+					return
+				}
+
+				if !yield(env) {
+					return
+				}
+
+			case <-ctx.Done():
+				return
+
+			case <-m.actorCtx.Done():
+				return
+			}
+		}
+	}
+}
+
+// Close implements Mailbox.Close.
+func (m *ChannelMailbox[M, R]) Close() {
+	m.closeOnce.Do(func() {
+		m.mu.Lock()
+		defer m.mu.Unlock()
+
+		m.closed.Store(true)
+
+		close(m.ch)
+	})
+}
+
+// IsClosed implements Mailbox.IsClosed.
+func (m *ChannelMailbox[M, R]) IsClosed() bool {
+	return m.closed.Load()
+}
+
+// Drain implements Mailbox.Drain for cleanup after close.
+func (m *ChannelMailbox[M, R]) Drain() iter.Seq[envelope[M, R]] {
+	return func(yield func(envelope[M, R]) bool) {
+		// Only drain if closed.
+		if !m.IsClosed() {
+			return
+		}
+
+		// Drain all remaining messages from the channel.
+		for {
+			select {
+			case env, ok := <-m.ch:
+				// Channel closed, nothing left to drain.
+				if !ok {
+					return
+				}
+
+				if !yield(env) {
+					return
+				}
+			default:
+				// Channel empty, done draining.
+				return
+			}
+		}
+	}
+}