M2 (TDD): storage contract test + memory impl

- src/storage/contract.test.js: storage interface spec (19 assertions)
- src/storage/memory.js: in-process impl (Map + EventEmitter)
- src/storage/storage.test.js: runner, memory first

TDD: contract RED first, memory built to satisfy, 19/19 green.
Next impls (ws, firebase) run same contract.

src/storage/memory.js

@@ -0,0 +1,126 @@
+// memory.js — in-process storage impl. Test seed.
+// Map<docPath, data>. EventEmitter for subscribe.
+// Mirrors firebase semantics: setDoc=replace, updateDoc=shallow merge, addDoc=auto-id.
+
+'use strict';
+
+const { EventEmitter } = require('events');
+
+function createMemoryStorage() {
+  const docs = new Map();              // path -> data obj
+  const bus = new EventEmitter();
+  bus.setMaxListeners(1000);
+
+  // ---- path helpers ----
+  // collection path = path with even number of segments OR known collection.
+  // doc path = odd segments (coll/doc, coll/doc/subcoll/subdoc).
+  // getCollection(path) returns all docs whose path === path/id for any single id segment.
+  function isCollectionPath(p) {
+    return p.split('/').length % 2 === 1;
+  }
+
+  function emitDoc(path, data) { bus.emit('doc:' + path, data); }
+  function emitCollection(collPath) {
+    const children = collectionDocs(collPath);
+    bus.emit('coll:' + collPath, children);
+  }
+
+  function collectionDocs(collPath) {
+    const out = [];
+    const segLen = collPath.split('/').length + 1;
+    for (const [p, data] of docs) {
+      const segs = p.split('/');
+      if (segs.length !== segLen) continue;
+      const parent = segs.slice(0, -1).join('/');
+      if (parent === collPath) out.push(data);
+    }
+    return out;
+  }
+
+  function genId() {
+    return (typeof crypto !== 'undefined' && crypto.randomUUID)
+      ? crypto.randomUUID()
+      : `id_${Date.now()}_${Math.random().toString(36).slice(2, 10)}`;
+  }
+
+  const storage = {
+    async getDoc(path) {
+      return docs.has(path) ? deepClone(docs.get(path)) : null;
+    },
+
+    async setDoc(path, data) {
+      docs.set(path, deepClone(data));
+      emitDoc(path, deepClone(data));
+      // notify parent collection
+      const segs = path.split('/');
+      if (segs.length >= 2) emitCollection(segs.slice(0, -1).join('/'));
+    },
+
+    async updateDoc(path, patch) {
+      const existing = docs.has(path) ? docs.get(path) : {};
+      const merged = { ...existing, ...patch };
+      docs.set(path, merged);
+      emitDoc(path, deepClone(merged));
+      const segs = path.split('/');
+      if (segs.length >= 2) emitCollection(segs.slice(0, -1).join('/'));
+    },
+
+    async deleteDoc(path) {
+      docs.delete(path);
+      emitDoc(path, null);
+      const segs = path.split('/');
+      if (segs.length >= 2) emitCollection(segs.slice(0, -1).join('/'));
+    },
+
+    async addDoc(collectionPath, data) {
+      const id = genId();
+      const path = `${collectionPath}/${id}`;
+      docs.set(path, deepClone(data));
+      emitDoc(path, deepClone(data));
+      emitCollection(collectionPath);
+      return { id, path };
+    },
+
+    async getCollection(collPath) {
+      return collectionDocs(collPath).map(deepClone);
+    },
+
+    async batchWrite(ops) {
+      for (const op of ops) {
+        if (op.type === 'set') await storage.setDoc(op.path, op.data);
+        else if (op.type === 'delete') await storage.deleteDoc(op.path);
+        else if (op.type === 'update') await storage.updateDoc(op.path, op.data);
+      }
+    },
+
+    subscribeDoc(path, cb) {
+      // fire immediately with current value
+      const cur = docs.has(path) ? deepClone(docs.get(path)) : null;
+      Promise.resolve().then(() => cb(cur));
+      const handler = (data) => cb(data);
+      bus.on('doc:' + path, handler);
+      return () => bus.off('doc:' + path, handler);
+    },
+
+    subscribeCollection(collPath, cb) {
+      Promise.resolve().then(() => cb(collectionDocs(collPath).map(deepClone)));
+      const handler = (docs) => cb(docs);
+      bus.on('coll:' + collPath, handler);
+      return () => bus.off('coll:' + collPath, handler);
+    },
+
+    dispose() { bus.removeAllListeners(); docs.clear(); },
+
+    // test/debug
+    _docs: docs,
+  };
+
+  return storage;
+}
+
+function deepClone(v) {
+  if (v === null || v === undefined) return v;
+  return JSON.parse(JSON.stringify(v));
+}
+
+module.exports = { createMemoryStorage };