docs(abstract-eth): specify SD Trust as signer for EVM cross-chain recovery backing wallets

modules/abstract-eth/README.md

@@ -1 +1,57 @@
 # BitGo abstract-eth
+
+Abstract base implementation for EVM-compatible coins (ETH, ETC, Polygon, OP-ETH, Arb-ETH, etc.).
+
+## EVM Cross-Chain Recovery
+
+EVM cross-chain recovery allows funds accidentally sent to a wrong EVM chain to be
+recovered. The recovery flow produces an unsigned (cold/custody wallet) or half-signed
+(hot wallet) transaction JSON that must be signed by the appropriate trust team before
+it can be broadcast.
+
+### Trust Team Signing Responsibility
+
+For backing wallets under EVM cross-chain recovery:
+
+| Step | Responsible party | Action |
+|------|-------------------|--------|
+| 1 | Support engineer | Generate unsigned recovery JSON via `recoverEthLikeforEvmBasedRecovery` and provide it to SD Trust |
+| 2 | **SD (South Dakota) Trust** | Sign the unsigned JSON using the **Singapore key shards** and return the half-signed JSON |
+| 3 | Support engineer | Broadcast the half-signed transaction |
+
+**Key points to avoid ambiguity:**
+
+- **SD Trust** (South Dakota) is the team that receives the unsigned JSON and performs
+  the signing operation.
+- **SG Custody** (Singapore) does **not** sign directly and will not see a signing
+  request appear in their reports. SG Custody's role is to hold the key shards that
+  SD Trust must select when signing.
+- SD Trust must explicitly select the **Singapore key shards** when performing the
+  signing step. Using other shards will produce an invalid signature for wallets
+  custodied under Singapore.
+
+### Required Fields in the Unsigned JSON
+
+The unsigned transaction JSON must include the following fields before it is sent to
+SD Trust for signing:
+
+```json
+{
+  "nextContractSequenceId": <number>,
+  "expireTime": <unix-timestamp>
+}
+```
+
+If these fields are missing, SD Trust's signing tool will return an error. The
+`expireTime` should be set to a future Unix timestamp (typically one week out).
+
+### Half-Signed JSON Validation
+
+After SD Trust returns the half-signed JSON, verify that `halfSigned.txHex` is
+present. If it is missing the broadcast step will fail with:
+
+```
+Error: Invalid half signed recovery file, missing halfSigned.txHex
+```
+
+In that case, request a new signing from SD Trust using the updated unsigned JSON.

modules/abstract-eth/src/abstractEthLikeNewCoins.ts

@@ -1728,6 +1728,13 @@ export abstract class AbstractEthLikeNewCoins extends AbstractEthLikeCoin {
    * half-signed (for hot wallet) evm cross chain recovery transaction with
    * same expected arguments as recover method.
    * This helps recover funds from evm based wrong chain.
+   *
+   * Signing responsibility for backing wallets:
+   * - SD (South Dakota) Trust is responsible for signing the unsigned JSON
+   *   produced by this method and returning the half-signed JSON to support.
+   * - SD Trust must use the Singapore key shards when signing.
+   * - SG (Singapore) Custody Trust does not sign directly; its role is
+   *   only to hold the relevant key shards used by SD Trust.
    * @param {RecoverOptions} params
    * @returns {Promise<RecoveryInfo | OfflineVaultTxInfo>}
    */