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/storage.test.js

@@ -0,0 +1,8 @@
+// Runner: executes storage contract against each impl.
+// TDD: contract = spec. Run against memory first. RED until memory.js built.
+'use strict';
+
+const { runStorageContract } = require('./contract.test');
+const { createMemoryStorage } = require('./memory');
+
+runStorageContract('memory', () => createMemoryStorage());