Add MongoDB reachability validation to IPL sequence

- Add rec_services config section to beds.toml and test fixture
- Add RecNodeConfig struct; export from config module
- Add mongo::validate() and validate_all() — TCP ping per configured REC node
- Wire mongo::validate_all() into ipl() with env-aware error handling
- Add mongodb crate dependency (sync feature)
- Add unit tests for mongo validate error paths (closed port, bad address)
- Update README status table and project structure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -88,6 +88,7 @@ rustybeds/
 │   │   ├── mod.rs          # Loader — load() and load_from() for testability
 │   │   └── structs.rs      # Typed config structs (serde Deserialize)
 │   ├── amqp.rs             # RabbitMQ transport — validate(), future channel/queue ops
+│   ├── mongo.rs            # MongoDB transport — validate_all(), future adapter ops
 │   ├── lib.rs              # Public API surface for integration test harness
 │   ├── logging.rs          # tracing + journald init
 │   └── main.rs             # ipl() sequence + main()
@@ -137,7 +138,7 @@ The `config` crate deep-merges these at startup. Only keys present in the env fi
 | IPL sequence with env-aware error handling | Done |
 | RabbitMQ reachability validation | Done |
 | Unit test scaffolding + config fixture pattern | Done |
-| MongoDB reachability validation | Next |
+| MongoDB reachability validation | Done |
 | MariaDB reachability validation | Next |
 | Shared filesystem validation | Next |
 | AMQP channel / queue declaration | Planned |