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.

README.md

@@ -0,0 +1,18 @@
+# stalwart-migrator
+
+In-place upgrade tool for Stalwart Mail Server: 0.15.5 → latest, with no
+data loss, checkpointed rollback at every step, and automated post-migration
+validation.
+
+Design status: architected, not yet implemented — see
+[ARCHITECTURE.md](ARCHITECTURE.md) for the full design, the phase-by-phase
+plan, and the research it's grounded in.
+
+```
+stalwart-migrate preflight   # read-only checks + plan
+stalwart-migrate run         # execute the migration
+stalwart-migrate status <id>
+stalwart-migrate rollback <id>
+stalwart-migrate confirm <id>
+stalwart-migrate report <id>
+```