M1: contracts and vectors (SPEC v0.1, schema 1)

contracts/openapi.yaml (OpenAPI 3.1, E-1..E-33, error shape and the
exhaustive error code enum, device bearer scheme), contracts/schema/
server-v1.sql (SPEC §14.1 verbatim), seven test-vector files per §14.5.5,
80 examples (every JSON body of §14.3.4, every error code, sync pull/push,
every FCM push type), and tools/contracts-check.py (stdlib-only reference
of §9.1, §9.3, §9.4 and §14.5.4 plus shape checks; the M1 gate).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

contracts/examples/push/sync_batch.json

@@ -0,0 +1,17 @@
+{
+  "message": {
+    "token": "dGhpcyBpcyBhbiBleGFtcGxlIGZjbSB0b2tlbg",
+    "android": {
+      "priority": "high",
+      "ttl": "86400s"
+    },
+    "data": {
+      "v": "1",
+      "type": "sync_batch",
+      "entity_type": "",
+      "entity_id": "",
+      "actor_user_id": "0192a000-0000-7000-8000-000000000002",
+      "count": "4"
+    }
+  }
+}