Add msgpack wire protocol with halfsiphash checksums

lib/wire/wire.go

@@ -0,0 +1,40 @@
+package wire
+
+import (
+	"fmt"
+
+	"github.com/theater/picomap/lib/halfsiphash"
+	"github.com/theater/picomap/lib/msgpack"
+)
+
+var HashKey = [8]byte{}
+
+type RebootingBootsel struct{}
+
+type Envelope struct {
+	Checksum uint32
+	Payload  []byte
+}
+
+func init() {
+	msgpack.RegisterExt(0, (*Envelope)(nil))
+	msgpack.RegisterExt(1, (*RebootingBootsel)(nil))
+}
+
+func DecodeMessage(data []byte) (any, error) {
+	var env Envelope
+	if err := msgpack.Unmarshal(data, &env); err != nil {
+		return nil, fmt.Errorf("decode envelope: %w", err)
+	}
+
+	expected := halfsiphash.Sum32(env.Payload, HashKey)
+	if env.Checksum != expected {
+		return nil, fmt.Errorf("checksum mismatch: got %08x, want %08x", env.Checksum, expected)
+	}
+
+	var inner any
+	if err := msgpack.Unmarshal(env.Payload, &inner); err != nil {
+		return nil, fmt.Errorf("decode inner: %w", err)
+	}
+	return inner, nil
+}