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>

bun.lock

@@ -63,6 +63,19 @@
         "@shade/core": "workspace:*",
       },
     },
+    "packages/shade-sdk": {
+      "name": "@shade/sdk",
+      "version": "0.1.0",
+      "dependencies": {
+        "@shade/core": "workspace:*",
+        "@shade/crypto-web": "workspace:*",
+        "@shade/observer": "workspace:*",
+        "@shade/proto": "workspace:*",
+        "@shade/server": "workspace:*",
+        "@shade/storage-sqlite": "workspace:*",
+        "@shade/transport": "workspace:*",
+      },
+    },
     "packages/shade-server": {
       "name": "@shade/server",
       "version": "0.1.0",
@@ -288,6 +301,8 @@
 
     "@shade/proto": ["@shade/proto@workspace:packages/shade-proto"],
 
+    "@shade/sdk": ["@shade/sdk@workspace:packages/shade-sdk"],
+
     "@shade/server": ["@shade/server@workspace:packages/shade-server"],
 
     "@shade/storage-postgres": ["@shade/storage-postgres@workspace:packages/shade-storage-postgres"],