feat(gateway): OpenAI-compatible custodial gateway

New multi-tenant gateway package: external developers point any OpenAI SDK at
the marketplace. Each API key is its own custodial buyer wallet (AES-256-GCM
sealed under GATEWAY_MASTER_KEY); every request posts an on-chain escrow from
that user's wallet.

- POST /openai/v1/chat/completions (one escrow/call, llm.text.generate.v1,
  buffered + pseudo-stream), chat sessions (one escrow/session, llm.chat.v1,
  real SSE), GET /openai/v1/models, /signup, /account, /account/withdraw.
- Settles by resolving the Submitted ref via the indexer + awaited Accept;
  state-aware reclaim sweeper + per-wallet auto-consolidation for collateral.
- Reuses the buyer Marketplace SDK via new buyer exports ./sdk; new shared
  buildWithdrawTx (custodial exit). Runs via tsx; Dockerfile + mainnet compose.

Design in docs/gateway.md. Typecheck passes for all packages; 20 vitest tests
pass. Not yet exercised against a live chain.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/shared/src/tx/server.ts

@@ -38,3 +38,10 @@ export type {
   ConsolidateWalletFlowResult,
 } from "./wallet/runConsolidateWallet.js";
 export { runConsolidateWallet } from "./wallet/runConsolidateWallet.js";
+
+// ── Wallet withdraw (custodial exit; live-only) ─────────────────────────
+export type {
+  WithdrawWalletParams,
+  WithdrawWalletBuildResult,
+} from "./wallet/withdrawWallet.js";
+export { buildWithdrawTx } from "./wallet/withdrawWallet.js";

packages/shared/src/tx/wallet/withdrawWallet.ts

@@ -0,0 +1,81 @@
+/**
+ * withdrawWallet.ts — custodial-exit tx builder (wallet → external address).
+ *
+ * Mirrors consolidateWallet.ts but pays a chosen amount to an arbitrary bech32
+ * address (lucid returns the change to the wallet). This is the withdrawal path
+ * for the gateway's custodial wallets: a user can move their unspent AP3X back
+ * out. No script, no datum, no native assets.
+ *
+ * `amountLovelace` omitted ⇒ withdraw all available minus a fee reserve.
+ * Live-only (needs a real Ogmios endpoint).
+ */
+
+import type { LiveOgmiosProvider } from "../../chain/LiveOgmiosProvider.js";
+import type { BuildResult, WalletKey } from "../types.js";
+import { TxConstructionError } from "../types.js";
+import { OgmiosLucidProvider } from "../../chain/OgmiosLucidProvider.js";
+import { createLucidContext } from "../internal/lucidContext.js";
+import { DEFAULT_FEE_RESERVE } from "./planConsolidate.js";
+
+export interface WithdrawWalletParams {
+  chain: LiveOgmiosProvider;
+  walletKey: WalletKey;
+  /** Destination bech32 address. */
+  toAddress: string;
+  /** Lovelace to send. Omit to withdraw all available minus the fee reserve. */
+  amountLovelace?: bigint;
+}
+
+export interface WithdrawWalletBuildResult extends BuildResult {
+  amountLovelace: bigint;
+  toAddress: string;
+}
+
+export async function buildWithdrawTx(
+  params: WithdrawWalletParams,
+): Promise<WithdrawWalletBuildResult> {
+  const { chain, walletKey, toAddress } = params;
+
+  const provider = new OgmiosLucidProvider({ ogmiosUrl: chain.url, fetch: chain.fetchImpl });
+  const { lucid } = await createLucidContext(
+    provider,
+    walletKey,
+    { networkId: 1, systemStartUnix: 0, slotLengthMs: 1000 },
+    { usePresetProtocolParameters: true },
+  );
+
+  const realWalletUtxos = await lucid.wallet().getUtxos();
+  const total = realWalletUtxos.reduce((sum, u) => sum + (u.assets.lovelace ?? 0n), 0n);
+  const spendable = total - DEFAULT_FEE_RESERVE;
+
+  const amount = params.amountLovelace ?? spendable;
+  if (amount <= 0n) {
+    throw new TxConstructionError("withdraw_amount_invalid", "nothing to withdraw");
+  }
+  if (amount > spendable) {
+    throw new TxConstructionError(
+      "withdraw_insufficient",
+      `requested ${amount} exceeds withdrawable ${spendable} (total ${total} minus fee reserve)`,
+    );
+  }
+
+  let signed;
+  try {
+    const txBuilder = lucid
+      .newTx()
+      .pay.ToAddress(toAddress, { lovelace: amount })
+      .addSignerKey(walletKey.pubKeyHash)
+      .setMinFee(500_000n);
+    const completed = await txBuilder.complete({ presetWalletInputs: realWalletUtxos, localUPLCEval: false });
+    signed = await completed.sign.withWallet().complete();
+  } catch (err) {
+    const msg = err instanceof Error ? err.message : String(err);
+    throw new TxConstructionError("withdraw_build_failed", msg);
+  }
+
+  const txCborHex = signed.toCBOR();
+  const expectedTxHash = signed.toHash();
+  await chain.submitTx(txCborHex);
+
+  return { txCborHex, expectedTxHash, amountLovelace: amount, toAddress };
+}