CSHLD-1700: Build Ironwood witness auth-path from shard data in wasm-utxo

packages/wasm-utxo/Cargo.toml

@@ -73,6 +73,12 @@ postcard = { version = "1.0", default-features = false, features = ["use-std"] }
 # `ff::PrimeField::to_repr` for the one PCZT witness field (`alpha`, a Pallas scalar) that orchard
 # exposes only as a curve scalar. Already in-tree via orchard/pasta_curves.
 ff = "0.13"
+# incrementalmerkletree::{Hashable, Level} — needed to fold a shard's leaves up to its local root
+# (empty_leaf/empty_root/combine) in build_ironwood_witness_from_shard. orchard's public API does
+# NOT re-export these (only MerklePath/MerkleHashOrchard), so this must be a direct dependency.
+# Pinned to match orchard 0.15.0's own resolved pin (see Cargo.lock) so MerkleHashOrchard's
+# `impl Hashable` resolves against the same crate version.
+incrementalmerkletree = "0.8"
 
 # Pinned to avoid RUSTSEC-2026-0204 (invalid pointer dereference in fmt::Pointer)
 crossbeam-epoch = ">=0.9.20"

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

@@ -1,6 +1,7 @@
 import {
   IronwoodWitness as WasmIronwoodWitness,
   ironwood_build_witness,
+  ironwood_build_witness_from_shard,
 } from "../wasm/wasm_utxo.js";
 
 /**
@@ -31,6 +32,24 @@ export class ZcashIronwoodWitness {
     return new ZcashIronwoodWitness(ironwood_build_witness(cmx, position, authPath, anchor));
   }
 
+  /**
+   * Build and validate a Merkle witness from one shard's leaves plus the sibling path above it,
+   * instead of a caller-precomputed 32-entry path.
+   * @throws Under the same conditions as {@link build}, plus if `shardHeight`/`upperPath`/
+   *   `shardLeaves` are inconsistent with each other or don't cover `position`
+   */
+  static buildFromShard(
+    position: number,
+    shardHeight: number,
+    shardLeaves: Uint8Array[],
+    upperPath: Uint8Array[],
+    anchor: Uint8Array,
+  ): ZcashIronwoodWitness {
+    return new ZcashIronwoodWitness(
+      ironwood_build_witness_from_shard(position, shardHeight, shardLeaves, upperPath, anchor),
+    );
+  }
+
   /** The leaf's position in the note commitment tree. */
   get position(): number {
     return this._wasm.position;

packages/wasm-utxo/src/wasm/zcash.rs

@@ -114,6 +114,57 @@ pub fn ironwood_build_witness(
     })
 }
 
+/// Build and validate an Ironwood witness from one shard's leaves plus the sibling path above it,
+/// instead of a caller-precomputed 32-entry path (see `ironwood_build_witness`).
+///
+/// `shardHeight` chooses how large the shard is (up to `2^shardHeight` leaves); `shardLeaves` must
+/// be the dense, contiguous prefix of real leaves for that shard (index = shard-local position);
+/// `upperPath` must have exactly `32 - shardHeight` entries. Throws under the same conditions as
+/// `ironwood_build_witness`, plus if `shardHeight`/`upperPath`/`shardLeaves` are inconsistent with
+/// each other or don't cover `position`.
+#[wasm_bindgen]
+pub fn ironwood_build_witness_from_shard(
+    position: u32,
+    shard_height: u8,
+    shard_leaves: Vec<js_sys::Uint8Array>,
+    upper_path: Vec<js_sys::Uint8Array>,
+    anchor: &[u8],
+) -> Result<IronwoodWitness, WasmUtxoError> {
+    use crate::zcash::ironwood_build::{build_ironwood_witness_from_shard, ShardWitnessInput};
+
+    fn to_arrays(v: &[js_sys::Uint8Array], label: &str) -> Result<Vec<[u8; 32]>, WasmUtxoError> {
+        v.iter()
+            .enumerate()
+            .map(|(i, u)| {
+                u.to_vec().try_into().map_err(|_| {
+                    WasmUtxoError::new(&format!(
+                        "{label}[{i}] must be 32 bytes, got {}",
+                        u.length()
+                    ))
+                })
+            })
+            .collect()
+    }
+
+    let anchor: [u8; 32] = anchor.try_into().map_err(|_| {
+        WasmUtxoError::new(&format!("anchor must be 32 bytes, got {}", anchor.len()))
+    })?;
+    let shard_leaves = to_arrays(&shard_leaves, "shardLeaves")?;
+    let upper_path = to_arrays(&upper_path, "upperPath")?;
+
+    Ok(IronwoodWitness {
+        inner: build_ironwood_witness_from_shard(
+            &ShardWitnessInput {
+                position,
+                shard_height,
+                shard_leaves: &shard_leaves,
+                upper_path: &upper_path,
+            },
+            &anchor,
+        )?,
+    })
+}
+
 /// Resolve the Orchard/Ironwood receiver of a ZIP-316 unified address for `coin`'s network, as
 /// its raw 43 bytes (diversifier + `pk_d`) — there is no scriptPubKey for a shielded output, so
 /// this can't return script bytes uniformly and returns raw receiver bytes instead.