phase 2.1-2.4: send path — submit + status, txbuilder, wallet.send, wallet.tx_status

chain backend grew submit_tx (POST /submittx, raw cbor body) and
tx_status (POST /tx_info → Confirmed{block,epoch}|NotFound). serde
tag-based status enum so the mcp tool returns clean json.

new core::tx module: ProtocolParams + InputUtxo + build_signed_payment.
two-pass fee refinement — build unsigned, measure size, add witness
overhead constant (128 bytes for vkey+sig+cbor framing), recompute
real fee, build with final fee, sign once (PrivateKey doesn't impl
Clone in pallas-wallet, so we don't double-sign). change below
min-utxo merges into fee instead of emitting dust.

added pallas-txbuilder + pallas-wallet 0.32 deps. PaymentKey gains
crate-private xprv() accessor; payment_key_to_private converts
ed25519-bip32 XPrv → pallas-wallet PrivateKey::Extended via the
64-byte extended secret bytes.

mcp tools.rs: 4 → 6 tools.
- wallet.send (to_address, lovelace, force) with hard-cap guard
- wallet.tx_status (tx_hash) → status json
SendArgs/TxStatusArgs use schemars derive so rmcp generates proper
input schemas. config.rs adds max_send_lovelace (default 100 ADA,
ALDABRA_MAX_SEND_LOVELACE env override).

37 unit tests. mcp tools/list smoke confirms all 6 tools register
with correct schemas (force defaults false, lovelace required uint64,
to_address required string).

phase 2.5 (native-asset send), 2.6 (cold-sign offline mode), and
2.7 (real preprod smoke against a funded wallet) still open.

crates/aldabra-chain/src/lib.rs

@@ -55,11 +55,39 @@ pub struct Balance {
     pub assets: std::collections::BTreeMap<String, u64>,
 }
 
+/// Confirmation status of a submitted transaction.
+///
+/// `NotFound` covers two cases the chain backend can't easily
+/// distinguish: the tx is in some mempool but not yet indexed by
+/// Koios, or it was never submitted / was rejected. Treat
+/// `NotFound` as "keep polling" up to a reasonable timeout, after
+/// which give up.
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(tag = "status", rename_all = "snake_case")]
+pub enum TxStatus {
+    /// Confirmed on-chain. `block_height` and `epoch` are populated
+    /// when Koios returns them.
+    Confirmed {
+        block_height: Option<u64>,
+        epoch: Option<u64>,
+    },
+    /// Not (yet) seen by the chain backend.
+    NotFound,
+}
+
 #[async_trait::async_trait]
 pub trait ChainBackend: Send + Sync {
+    /// All UTXOs at the given address.
     async fn get_utxos(&self, address: &str) -> Result<Vec<Utxo>, ChainError>;
+
+    /// Aggregated ADA + native-asset balance at the address.
     async fn get_balance(&self, address: &str) -> Result<Balance, ChainError>;
-    // Phase 2:
-    // async fn submit_tx(&self, raw_tx_cbor: &[u8]) -> Result<String, ChainError>;
-    // async fn tx_status(&self, tx_hash: &str) -> Result<TxStatus, ChainError>;
+
+    /// Submit a signed transaction. `raw_tx_cbor` is the binary CBOR
+    /// payload of the signed tx; on success returns the tx hash as
+    /// a hex string.
+    async fn submit_tx(&self, raw_tx_cbor: &[u8]) -> Result<String, ChainError>;
+
+    /// Poll the chain backend for a tx's current confirmation status.
+    async fn tx_status(&self, tx_hash: &str) -> Result<TxStatus, ChainError>;
 }