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

server/crates/arbiter-server/src/peers/operator/session/handlers.rs

@@ -335,3 +335,46 @@ impl OperatorSession {
             .unwrap_or_default()
     }
 }
+
+#[messages]
+impl OperatorSession {
+    #[message]
+    pub(crate) async fn handle_contribute_rekey_passphrase(
+        &mut self,
+        passphrase: Vec<u8>,
+    ) -> Result<bool, Error> {
+        use crate::actors::vault_coordinator::ContributeRekey;
+        use arbiter_crypto::safecell::{SafeCell, SafeCellHandle as _};
+
+        let operator_id = self.credentials.id;
+        self.props
+            .actors
+            .vault_coordinator
+            .ask(ContributeRekey {
+                operator_id,
+                passphrase: SafeCell::new(passphrase),
+            })
+            .await
+            .map_err(|_| Error::internal("VaultCoordinator unavailable"))
+    }
+
+    #[message]
+    pub(crate) async fn handle_contribute_recovery_rekey_passphrase(
+        &mut self,
+        recovery_operator_id: i32,
+        passphrase: Vec<u8>,
+    ) -> Result<bool, Error> {
+        use crate::actors::vault_coordinator::ContributeRecoveryRekey;
+        use arbiter_crypto::safecell::{SafeCell, SafeCellHandle as _};
+
+        self.props
+            .actors
+            .vault_coordinator
+            .ask(ContributeRecoveryRekey {
+                recovery_operator_id,
+                passphrase: SafeCell::new(passphrase),
+            })
+            .await
+            .map_err(|_| Error::internal("VaultCoordinator unavailable"))
+    }
+}