feat(vault)!: implement full Shamir re-key flow and governance execution (§3.3–§3.5)

- Add `rekey.proto` with `ContributePassphrase` / `ContributeRecoveryPassphrase` / `RekeyResult`
- Wire `rekey` as a 4th vault stream payload in `vault.proto` and gRPC dispatch
- Add `RekeyRootKey` message to `Vault` actor: generates new random seal key, re-encrypts root key, writes new `root_key_history` row
- Add `StartRekey`, `ContributeRekey`, `ContributeRecoveryRekey` messages to `VaultCoordinator`; `finalize_rekey` uses threshold-1 fast path identical to bootstrap
- `execute_replace_operator` now UPDATEs `operator_identity.public_key` in-place (avoids FK constraint violation), deletes stale `operator` share row, then triggers `StartRekey`
- `execute_update_shamir_parameters` triggers `StartRekey` instead of warning stub
- `ProposalKind::ReplaceOperator` carries `old_operator_id`; encode/decode updated accordingly
- `GlobalActors::spawn` extracts `vault_coordinator` before `Ok(Self { … })` so it can be cloned into `ProposalManager::new`
- Add `handle_rekey` in session handlers forwarding passphrase contributions to `VaultCoordinator`
- Fix test: rename `replace_operator_inserts_identity_row` → `replace_operator_updates_pubkey_and_starts_rekey`, assert count stays 1 and pubkey is updated

protobufs/operator/vault/rekey.proto

@@ -0,0 +1,30 @@
+syntax = "proto3";
+
+package arbiter.operator.vault.rekey;
+
+message ContributePassphrase {
+  bytes passphrase = 1;
+}
+
+message ContributeRecoveryPassphrase {
+  int32 recovery_operator_id = 1;
+  bytes passphrase           = 2;
+}
+
+enum RekeyResult {
+  REKEY_RESULT_UNSPECIFIED            = 0;
+  REKEY_RESULT_SUCCESS                = 1;
+  REKEY_RESULT_AWAITING_CONTRIBUTIONS = 2;
+  REKEY_RESULT_NOT_IN_PROGRESS        = 3;
+}
+
+message Request {
+  oneof payload {
+    ContributePassphrase         contribute_passphrase          = 1;
+    ContributeRecoveryPassphrase contribute_recovery_passphrase = 2;
+  }
+}
+
+message Response {
+  RekeyResult result = 1;
+}

protobufs/operator/vault/vault.proto

@@ -5,20 +5,23 @@ package arbiter.operator.vault;
 import "google/protobuf/empty.proto";
 import "shared/vault.proto";
 import "operator/vault/bootstrap.proto";
+import "operator/vault/rekey.proto";
 import "operator/vault/unseal.proto";
 
 message Request {
   oneof payload {
     google.protobuf.Empty query_state = 1;
-    unseal.Request unseal = 2;
-    bootstrap.Request bootstrap = 3;
+    unseal.Request        unseal      = 2;
+    bootstrap.Request     bootstrap   = 3;
+    rekey.Request         rekey       = 4;
   }
 }
 
 message Response {
   oneof payload {
-    arbiter.shared.VaultState state = 1;
-    unseal.Response unseal = 2;
-    bootstrap.Response bootstrap = 3;
+    arbiter.shared.VaultState state     = 1;
+    unseal.Response           unseal    = 2;
+    bootstrap.Response        bootstrap = 3;
+    rekey.Response            rekey     = 4;
   }
 }