docs: add architecture migration guardrails (#3253)

docs/architecture/compat-cleanup-register.md

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+# Compatibility Cleanup Register
+
+Use this file to track temporary compatibility code introduced by architecture
+migration PRs. Entries are required only for compatibility paths that are planned
+for later deletion.
+
+## Required Source Marker
+
+```rust
+// RUSTFS_COMPAT_TODO(<task-id>): <why this compatibility path exists>. Remove after <specific condition>.
+```
+
+## Open Items
+
+No compatibility code has been introduced in this branch.
+
+## Review Checklist
+
+Before completing a PR that adds wrappers, re-exports, fallbacks, legacy action
+mappings, or old endpoint compatibility layers:
+
+- [ ] The source has a `RUSTFS_COMPAT_TODO(<task-id>)` marker.
+- [ ] This register has a matching entry.
+- [ ] The entry states why compatibility is needed.
+- [ ] The entry states the exact removal condition.
+- [ ] The cleanup is not bundled with new migration logic.