refactor: move explain logic out of wasm-dot, accept metadata as hex string

- remove explainTransaction and explain.ts from wasm-dot (moved to BitGoJS)
- narrow parseTransaction to accept only DotTransaction (not raw bytes/hex)
- add fromHex, toHex, toBroadcastFormat to DotTransaction TS wrapper
- pass material at deserialization time (fromHex/fromBytes) so metadata-
  aware signed extension parsing can determine call_data boundaries
- revert Material.metadata from Uint8Array back to string (0x-prefixed hex)
  to match the Substrate RPC wire format and avoid forcing callers to convert
- consolidate decode_metadata into a single pub(crate) function in
  transaction.rs, remove duplicates from parser.rs and builder/mod.rs

BTC-3093

Author: lcovar

package-lock.json

@@ -2,8 +2,7 @@
  * wasm-dot: WASM bindings for Polkadot/DOT transaction operations
  *
  * This module provides:
- * - Transaction parsing: parseTransaction(bytes, context) → ParsedTransaction
- * - Transaction explanation: explainTransaction(bytes, options) → ExplainedTransaction
+ * - Transaction parsing: parseTransaction(tx, context) → ParsedTransaction
  * - Transaction building: buildTransaction(intent, context) → DotTransaction
  * - Transaction signing: DotTransaction.fromBytes(bytes) → inspect + sign
  */
@@ -32,4 +31,3 @@ export * from "./types";
 export * from "./transaction";
 export * from "./parser";
 export * from "./builder";
-export * from "./explain";

packages/wasm-dot/js/explain.ts

@@ -1,5 +1,5 @@
 /**
- * Transaction parsing — standalone function that decodes extrinsic bytes
+ * Transaction parsing — standalone function that decodes a DotTransaction
  * into structured data (pallet, method, args, nonce, etc.).
  *
  * This is separate from the DotTransaction class, which handles signing.
@@ -8,51 +8,35 @@
  */
 
 import { ParserNamespace, MaterialJs, ParseContextJs } from "./wasm/wasm_dot";
-import { DotTransaction } from "./transaction";
+import type { DotTransaction } from "./transaction";
 import type { ParseContext, ParsedTransaction } from "./types";
 
-/**
- * Input type for parsing — accepts raw bytes, hex string, or DotTransaction.
- */
-export type TransactionInput = Uint8Array | string | DotTransaction;
-
 /**
  * Parse a DOT transaction into structured data.
  *
+ * Accepts a `DotTransaction` object (from `DotTransaction.fromBytes()`),
+ * avoiding double deserialization.
+ *
  * Returns a plain `ParsedTransaction` object with decoded pallet, method,
  * args, nonce, tip, era, etc.
  *
- * For a signable `DotTransaction` object, use `DotTransaction.fromBytes()` instead.
- *
- * @param input - Raw bytes, hex string (with or without 0x), or DotTransaction
+ * @param tx - A DotTransaction instance (from DotTransaction.fromBytes())
  * @param context - Parsing context with chain material (required for decoding)
  * @returns Parsed transaction data
  *
  * @example
  * ```typescript
- * import { parseTransaction } from '@bitgo/wasm-dot';
+ * import { DotTransaction, parseTransaction } from '@bitgo/wasm-dot';
  *
- * const parsed = parseTransaction(txBytes, { material });
+ * const tx = DotTransaction.fromBytes(txBytes, context);
+ * const parsed = parseTransaction(tx, { material });
  * console.log(parsed.method.pallet); // "balances"
  * console.log(parsed.method.name);   // "transferKeepAlive"
  * ```
  */
-export function parseTransaction(
-  input: TransactionInput,
-  context?: ParseContext,
-): ParsedTransaction {
+export function parseTransaction(tx: DotTransaction, context?: ParseContext): ParsedTransaction {
   const ctx = context ? createParseContext(context) : undefined;
-
-  if (input instanceof DotTransaction) {
-    return ParserNamespace.parseTransaction(input.toBytes(), ctx) as ParsedTransaction;
-  }
-
-  if (input instanceof Uint8Array) {
-    return ParserNamespace.parseTransaction(input, ctx) as ParsedTransaction;
-  }
-
-  // String input — let WASM handle hex parsing
-  return ParserNamespace.parseTransactionHex(input, ctx) as ParsedTransaction;
+  return ParserNamespace.parseFromTransaction(tx.wasm, ctx) as ParsedTransaction;
 }
 
 /**