Add Zapier interface simulator, tree-node Merkle lookups, and user docs

In-process trace console validates hops, faults, recoveries, and
suggested changes before zapier-platform push. User guide covers
signup through central-chain and bulk-summary tree-node hash lookup.

packages/verae-tree-node/src/merkle.js

@@ -0,0 +1,96 @@
+/**
+ * Merkle tree over SHA-256 leaf hex digests.
+ * Only the root is sealed on the main Verae chain; leaves live on tree nodes.
+ * @module merkle
+ */
+
+import { createHash } from 'node:crypto';
+
+/**
+ * @param {string|Buffer|Uint8Array} data
+ * @returns {string} lowercase hex
+ */
+export function sha256Hex(data) {
+  const buf = Buffer.isBuffer(data) ? data : Buffer.from(String(data), 'utf8');
+  return createHash('sha256').update(buf).digest('hex');
+}
+
+/**
+ * Parent of two 32-byte hex children (left || right).
+ * Odd last child is duplicated (Bitcoin-style).
+ * @param {string} left
+ * @param {string} right
+ */
+export function parentHash(left, right) {
+  return createHash('sha256')
+    .update(Buffer.from(left, 'hex'))
+    .update(Buffer.from(right, 'hex'))
+    .digest('hex');
+}
+
+/**
+ * @param {string[]} leaves hex SHA-256 digests
+ * @returns {{ root: string, layers: string[][], leaves: string[] }}
+ */
+export function buildMerkle(leaves) {
+  if (!leaves.length) {
+    throw new Error('at least one leaf is required');
+  }
+  const layer0 = leaves.map((h) => String(h).toLowerCase());
+  const layers = [layer0];
+  let layer = layer0;
+  while (layer.length > 1) {
+    const next = [];
+    for (let i = 0; i < layer.length; i += 2) {
+      const left = layer[i];
+      const right = layer[i + 1] ?? layer[i];
+      next.push(parentHash(left, right));
+    }
+    layer = next;
+    layers.push(layer);
+  }
+  return { root: layer[0], layers, leaves: layer0 };
+}
+
+/**
+ * Inclusion proof for leaf index.
+ * @param {string[][]} layers
+ * @param {number} index
+ * @returns {Array<{ sibling: string, position: 'left'|'right' }>}
+ */
+export function merkleProof(layers, index) {
+  const proof = [];
+  let i = index;
+  for (let level = 0; level < layers.length - 1; level += 1) {
+    const layer = layers[level];
+    const isRight = i % 2 === 1;
+    const sib = isRight ? i - 1 : i + 1;
+    const sibling = layer[sib] ?? layer[i];
+    proof.push({ sibling, position: isRight ? 'left' : 'right' });
+    i = Math.floor(i / 2);
+  }
+  return proof;
+}
+
+/**
+ * @param {string} leaf
+ * @param {Array<{ sibling: string, position: 'left'|'right' }>} proof
+ * @param {string} root
+ */
+export function verifyProof(leaf, proof, root) {
+  let h = String(leaf).toLowerCase();
+  for (const step of proof) {
+    h = step.position === 'left' ? parentHash(step.sibling, h) : parentHash(h, step.sibling);
+  }
+  return h === String(root).toLowerCase();
+}
+
+/**
+ * Shard a leaf onto one of N tree nodes (others stay silent on bloom miss).
+ * @param {string} sha256
+ * @param {number} n
+ */
+export function shardIndex(sha256, n) {
+  const b = Buffer.from(String(sha256).slice(0, 8), 'hex');
+  return n <= 0 ? 0 : b.readUInt32BE(0) % n;
+}