feat(sdk): M-Magic 1-4 — high-level SDK with magic drop-in

Phase A complete: createShade() one-liner with auto-establish, auto-publish,
and auto-replenish.

M-Magic 1-4 rolled into @shade/sdk:
- createShade() factory with config validation and storage resolution
  (memory | sqlite:... | { type: 'postgres', url: ... } | explicit instance)
- Shade class wraps crypto + storage + session manager + transport
- Auto-publish: initialize() automatically registers with the prekey server
- Auto-establish: send() transparently fetches bundles and creates sessions
  on first message to a new peer
- Per-address mutex serializes concurrent sends to prevent ratchet corruption
- BackgroundTasks class for periodic replenishment + opt-in identity rotation
- rotate() rebuilds the transport with the new signing key so subsequent
  signed operations work after rotation
- onMessage() handler API for incoming plaintext

API:
  const shade = await createShade({ prekeyServer, storage });
  await shade.send('bob', 'hello');
  await shade.receive('alice', envelope);
  shade.onMessage((from, msg) => ...);
  await shade.rotate();
  await shade.shutdown();

13 new SDK tests covering: happy path, auto-publish, two-process
conversation, onMessage handlers, concurrent sends, unknown peer,
fingerprint verification, shutdown, manual replenish, auto-replenish
off, rotate, and config validation.

233 tests passing, 0 failures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shade-sdk/src/create-shade.ts

@@ -0,0 +1,23 @@
+import { resolveConfig, type ShadeConfig } from './config.js';
+import { Shade } from './shade.js';
+
+/**
+ * Create and initialize a Shade instance in one call.
+ *
+ * ```ts
+ * const shade = await createShade({
+ *   prekeyServer: 'https://shade.example.com',
+ *   storage: 'sqlite:/data/shade.db',
+ * });
+ *
+ * await shade.send('bob@example.com', 'hello');
+ * ```
+ *
+ * See ShadeConfig for all options.
+ */
+export async function createShade(config: ShadeConfig): Promise<Shade> {
+  const resolved = resolveConfig(config);
+  const shade = new Shade(resolved);
+  await shade.initialize();
+  return shade;
+}