staticaddr/deposit: document lock ordering

Document the lock-order invariant between Manager.mu and individual
deposit locks.

Later changes need both locks in the same path, so make the rule
explicit before the locking surface grows.

staticaddr/deposit/deposit.go

@@ -29,6 +29,10 @@ func (r *ID) FromByteSlice(b []byte) error {
 
 // Deposit bundles an utxo at a static address together with manager-relevant
 // data.
+//
+// Lock order: if both Manager.mu and a Deposit lock are needed, acquire
+// Manager.mu before Deposit.Lock. Never acquire Manager.mu while holding a
+// Deposit lock.
 type Deposit struct {
 	sync.Mutex
 

staticaddr/deposit/manager.go

@@ -58,6 +58,10 @@ type ManagerConfig struct {
 }
 
 // Manager manages the address state machines.
+//
+// Lock order: if both Manager.mu and a Deposit lock are needed, acquire
+// Manager.mu before Deposit.Lock. Never acquire Manager.mu while holding a
+// Deposit lock.
 type Manager struct {
 	cfg *ManagerConfig