Initial commit: stalwart-migrator design and scaffolding

In-place upgrade tool for Stalwart Mail Server (0.15.5 -> latest) with
checkpointed rollback and post-migration validation. Design stage; see
ARCHITECTURE.md.

.gitignore

@@ -0,0 +1,26 @@
+# Binaries
+/bin/
+/dist/
+stalwart-migrate
+*.exe
+
+# Test / coverage
+*.out
+*.test
+
+# Local state & logs
+*.log
+*.db
+*.db-journal
+*.db-wal
+*.db-shm
+
+# Credentials / local config
+.env
+*.pem
+*.key
+
+# Editor / OS
+.DS_Store
+.idea/
+.vscode/