Add RabbitMQ validation, test scaffolding, and config refactor

- Extract ipl() from main() with env-aware error handling (fatal in prod, warn in dev)
- Add amqp::validate() — TCP reachability check for RabbitMQ at IPL
- Refactor config::load() into load() + load_from() for testability
- Add lib.rs to expose public API to integration test harness
- Add test fixture scaffolding: tests/fixtures/beds_test.toml, tests/common/mod.rs
- Add unit tests for amqp::validate() error paths (closed port, bad address)

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

src/amqp.rs

@@ -0,0 +1,86 @@
+//! # amqp.rs — RabbitMQ Transport Layer
+//!
+//! Manages all AMQP interactions for the BEDS node. At IPL, validates that
+//! the RabbitMQ broker is reachable before the node proceeds. Future phases
+//! will add channel acquisition, queue declaration, and message dispatch.
+//!
+//! ## Calling Agents
+//! - `ipl()` in main.rs — calls `validate()` during the IPL sequence
+//!
+//! ## Inputs
+//! - `BrokerServicesConfig` from the loaded BEDS configuration
+//!
+//! ## Outputs
+//! - `Ok(())` if the broker is reachable
+//! - `Err(String)` with host:port and OS error if the broker cannot be reached
+//!
+//! **Author:** mks
+//! **Version:** 1.0
+//!
+//! ## History
+//! * `2026-04-02` - mks - original coding
+
+use std::net::TcpStream;
+use std::time::Duration;
+
+use crate::config::BrokerServicesConfig;
+
+/// Validates that the RabbitMQ broker is reachable.
+///
+/// Opens a TCP connection to the configured broker host and port. Does not
+/// authenticate or open an AMQP channel — reachability only. The connection
+/// is closed immediately after a successful connect.
+///
+/// # Arguments
+///
+/// * `cfg` — broker services configuration block from `BedsConfig`
+///
+/// # Returns
+///
+/// `Ok(())` if the TCP handshake succeeds.
+/// `Err(String)` with a descriptive message if the broker cannot be reached
+/// or if the configured address is malformed.
+///
+/// # History
+///
+/// * `2026-04-02` - mks - original coding
+pub fn validate(cfg: &BrokerServicesConfig) -> Result<(), String> {
+    let addr_str = format!("{}:{}", cfg.app_server.host, cfg.app_server.port);
+
+    let addr: std::net::SocketAddr = addr_str
+        .parse()
+        .map_err(|e| format!("Invalid broker address {}: {}", addr_str, e))?;
+
+    TcpStream::connect_timeout(&addr, Duration::from_secs(5))
+        .map_err(|e| format!("RabbitMQ unreachable at {}: {}", addr_str, e))?;
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::config::load_from;
+
+    /// Loads the test fixture config. Panics if the fixture is missing or malformed.
+    fn test_cfg() -> crate::config::BedsConfig {
+        load_from("tests/fixtures/beds_test.toml", "")
+            .expect("test fixture beds_test.toml failed to load")
+    }
+
+    #[test]
+    fn validate_err_on_closed_port() {
+        // port 1 is reserved and always closed — guarantees a connection refusal
+        // without requiring any live service
+        let mut cfg = test_cfg();
+        cfg.broker_services.app_server.port = 1;
+        assert!(validate(&cfg.broker_services).is_err());
+    }
+
+    #[test]
+    fn validate_err_on_bad_address() {
+        let mut cfg = test_cfg();
+        cfg.broker_services.app_server.host = "not_a_valid_host!!!".to_string();
+        assert!(validate(&cfg.broker_services).is_err());
+    }
+}