feat: add WASM-based DOT transaction parsing via @bitgo/wasm-dot

Integrates @bitgo/wasm-dot into sdk-coin-dot for WASM-based transaction
parsing using Rust/subxt, replacing the JS txwrapper-polkadot path for
tdot (testnet) signed transactions.

## what changed

- **wasmParser.ts**: new module with `explainDotTransaction()` and
  `toJsonFromWasm()` that parse DOT extrinsics via WASM and map to
  BitGoJS TransactionExplanation / TxData formats. handles transfers,
  staking (bond, unbond, withdraw, chill, payout), proxy (add/remove),
  batch (nested + atomic), and transferAll.

- **transaction.ts toJson()**: signed tdot transactions now use the WASM
  path (`toJsonFromWasm`), which handles metadata-aware signed extension
  parsing (e.g. Westend's AuthorizeCall, StorageWeightReclaim). unsigned
  transactions still use the legacy txwrapper path.

- **webpack config**: added ESM alias for @bitgo/wasm-dot so browser
  builds use the fetch-based WASM loader instead of fs.readFileSync.

- **tests**: 72+ new test lines in dot.ts covering WASM explain for
  transfers and staking. 511-line byte comparison test suite validating
  WASM builder output matches legacy txwrapper output byte-for-byte
  across all transaction types (transfer, stake, unstake, withdraw,
  chill, proxy, batch).

- **withdrawUnstakedBuilder test fix**: numSlashingSpans assertion
  updated from string to number to match actual type.

## scope

WASM path is only active for tdot signed transactions, gated by
`this._coinConfig.name === 'tdot' && this._signedTransaction`. mainnet
dot remains on the legacy path until validation is complete.

BTC-0

TICKET: BTC-0

Author: lcovar

modules/sdk-coin-dot/package.json

@@ -43,6 +43,7 @@
     "@bitgo/sdk-core": "^36.33.2",
     "@bitgo/sdk-lib-mpc": "^10.9.0",
     "@bitgo/statics": "^58.29.0",
+    "@bitgo/wasm-dot": "^1.1.2",
     "@polkadot/api": "14.1.1",
     "@polkadot/api-augment": "14.1.1",
     "@polkadot/keyring": "13.5.6",

modules/sdk-coin-dot/src/dot.ts

@@ -58,14 +58,6 @@ export interface TransactionPrebuild {
   transaction: Interface.TxData;
 }
 
-export interface ExplainTransactionOptions {
-  txPrebuild: TransactionPrebuild;
-  publicKey: string;
-  feeInfo: {
-    fee: string;
-  };
-}
-
 export interface VerifiedTransactionParameters {
   txHex: string;
   prv: string;

modules/sdk-coin-dot/src/lib/index.ts

@@ -16,4 +16,6 @@ export { TransactionBuilderFactory } from './transactionBuilderFactory';
 export { SingletonRegistry } from './singletonRegistry';
 export { NativeTransferBuilder } from './nativeTransferBuilder';
 export { RemoveProxyBuilder } from './proxyBuilder';
+export { explainDotTransaction } from './wasmParser';
+export type { ExplainDotTransactionParams, DotWasmExplanation, DotInput } from './wasmParser';
 export { Interface, Utils };

modules/sdk-coin-dot/src/lib/transaction.ts

@@ -38,6 +38,7 @@ import {
 } from './iface';
 import { getAddress, getDelegateAddress } from './iface_utils';
 import utils from './utils';
+import { toJsonFromWasm } from './wasmParser';
 import BigNumber from 'bignumber.js';
 import { Vec } from '@polkadot/types';
 import { PalletConstantMetadataV14 } from '@polkadot/types/interfaces';
@@ -161,6 +162,20 @@ export class Transaction extends BaseTransaction {
     if (!this._dotTransaction) {
       throw new InvalidTransactionError('Empty transaction');
     }
+
+    // WASM path for signed tdot transactions — validates WASM parsing against production.
+    // Only for signed txs because toBroadcastFormat() returns the signed extrinsic (parseable).
+    // Unsigned txs return a signing payload (different format), so they use the legacy path.
+    if (this._coinConfig.name === 'tdot' && this._signedTransaction) {
+      return toJsonFromWasm({
+        txHex: this._signedTransaction,
+        material: utils.getMaterial(this._coinConfig),
+        senderAddress: this._sender,
+        coinConfigName: this._coinConfig.name,
+        referenceBlock: this._dotTransaction.blockHash,
+        blockNumber: Number(this._dotTransaction.blockNumber),
+      });
+    }
     const decodedTx = decode(this._dotTransaction, {
       metadataRpc: this._dotTransaction.metadataRpc,
       registry: this._registry,