ci: lock EventEnvelope byte contract

.github/workflows/freeze-envelope.yml

@@ -0,0 +1,15 @@
+name: Freeze Envelope Byte Contract
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  freeze-envelope:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - name: Assert EventEnvelope byte contract
+        run: cargo test -p vaultmesh-command-center test_event_envelope_canonicalization_bytes
+