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-mcp/src/main.rs

@@ -79,6 +79,8 @@ async fn run() -> Result<()> {
         cfg.account,
         cfg.index,
     )?;
+    let payment_key =
+        aldabra_core::derive_payment_key(&root, cfg.account, cfg.index);
     tracing::info!(%address, "derived base address");
 
     if bootstrap_only {
@@ -88,7 +90,13 @@ async fn run() -> Result<()> {
 
     // Hand off to the MCP server. From this point on stdin/stdout
     // belong to the JSON-RPC transport — no more eprintln-prompting.
-    let service = WalletService::new(cfg.network, address, cfg.koios_base);
+    let service = WalletService::new(
+        cfg.network,
+        address,
+        cfg.koios_base,
+        payment_key,
+        cfg.max_send_lovelace,
+    );
     let server = service
         .serve(stdio())
         .await