feat(wasm-utxo): expose ZEC branch ID APIs on ZcashBitGoPsbt

Adds consensus_branch_id() to the wasm_bindgen BitGoPsbt impl (Zcash
variant only; returns None for non-ZEC PSBTs). Adds a
zcash_branch_id_for_height() free function wrapping the existing Rust
branch_id_for_height, returning a Result so it throws on unrecognised
network strings ("zcash"/"zec" for mainnet, "zcashTest"/"tzec" for
testnet — anything else is an error).

TypeScript ZcashBitGoPsbt gains:
  consensusBranchId: number | undefined — reads the branch ID stored in
    the PSBT proprietary map (absent for v5 or non-ZEC PSBTs).
  static branchIdForHeight(network, height): number | undefined — returns
    the consensus branch ID active at a given chain height.

These APIs allow callers (e.g. ims-utxo parse path) to validate a PSBT's
consensus branch ID against the current chain height without importing
@bitgo-beta/utxo-lib.

Refs: T1-3519

Author: OttoAllmendinger

packages/wasm-utxo/js/fixedScriptWallet/ZcashBitGoPsbt.ts

@@ -1,4 +1,4 @@
-import { BitGoPsbt as WasmBitGoPsbt } from "../wasm/wasm_utxo.js";
+import { BitGoPsbt as WasmBitGoPsbt, zcash_branch_id_for_height } from "../wasm/wasm_utxo.js";
 import { type WalletKeysArg, RootWalletKeys } from "./RootWalletKeys.js";
 import { BitGoPsbt, type CreateEmptyOptions, type HydrationUnspent } from "./BitGoPsbt.js";
 import { ZcashTransaction, type ITransaction } from "../transaction.js";
@@ -264,6 +264,22 @@ export class ZcashBitGoPsbt extends BitGoPsbt {
     return this.wasm.expiry_height();
   }
 
+  /**
+   * Get the Zcash consensus branch ID stored in the PSBT proprietary map.
+   * Returns undefined for v5 PSBTs or PSBTs without the key.
+   */
+  get consensusBranchId(): number | undefined {
+    return this.wasm.consensus_branch_id();
+  }
+
+  /**
+   * Return the Zcash consensus branch ID active at `height` on `network`.
+   * Returns undefined if `height` is before Overwinter activation.
+   */
+  static branchIdForHeight(network: ZcashNetworkName, height: number): number | undefined {
+    return zcash_branch_id_for_height(network, height);
+  }
+
   /**
    * Extract the final Zcash transaction from a finalized PSBT
    *

packages/wasm-utxo/src/wasm/fixed_script_wallet/mod.rs

@@ -961,6 +961,17 @@ impl BitGoPsbt {
         }
     }
 
+    /// Get the Zcash consensus branch ID from the PSBT proprietary map (returns None for non-Zcash PSBTs)
+    pub fn consensus_branch_id(&self) -> Option<u32> {
+        use crate::fixed_script_wallet::bitgo_psbt::{
+            propkv::get_zec_consensus_branch_id, BitGoPsbt as InnerBitGoPsbt,
+        };
+        match &self.psbt {
+            InnerBitGoPsbt::Zcash(z, _) => get_zec_consensus_branch_id(&z.psbt),
+            _ => None,
+        }
+    }
+
     pub fn get_outputs_with_address(&self) -> Result<JsValue, WasmUtxoError> {
         crate::wasm::psbt::get_outputs_with_address_from_psbt(self.psbt.psbt(), self.psbt.network())
     }
@@ -1943,3 +1954,23 @@ impl BitGoPsbt {
 }
 
 impl_wasm_psbt_ops!(BitGoPsbt, psbt);
+
+/// Return the Zcash consensus branch ID active at `height` on `network`.
+///
+/// `network`: "zcash" / "zec" for mainnet, "zcashTest" / "tzec" for testnet.
+/// Returns `None` if `height` is before Overwinter activation.
+/// Throws if `network` is not a recognised Zcash network name.
+#[wasm_bindgen]
+pub fn zcash_branch_id_for_height(network: &str, height: u32) -> Result<Option<u32>, JsValue> {
+    let is_mainnet = match network {
+        "zcash" | "zec" => true,
+        "zcashTest" | "tzec" => false,
+        _ => {
+            return Err(JsValue::from_str(&format!(
+            "unknown Zcash network {:?}: expected \"zcash\", \"zec\", \"zcashTest\", or \"tzec\"",
+            network
+        )))
+        }
+    };
+    Ok(crate::zcash::branch_id_for_height(height, is_mainnet))
+}