Add rBroker + wBroker pool, BrokerPayload, NamasteCore trait stub

- src/brokers/: pool manager, r_broker (rec.read), w_broker (rec.write),
  BrokerPayload struct, BrokerError type
- src/core/: NamasteCore trait — fetch/write/update/delete interface, stubs
- IPL step 6: spawns rBroker + wBroker pools after exchange declaration
- tests/broker_pool_test.rs: integration tests for pool spawn (skip if broker down)
- BrokerPayload unit tests + doctest in payload.rs
- Added futures-lite, serde_json to Cargo.toml
- README.md, CLAUDE.md, wiki updated to reflect new structure and status

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

src/brokers/payload.rs

@@ -0,0 +1,108 @@
+//! # brokers/payload.rs — AMQP Message Payload
+//!
+//! Defines the JSON body structure carried in all BEDS broker messages.
+//! The AMQP envelope handles routing (type header, reply_to, correlation_id);
+//! this struct is what lives in the message body.
+//!
+//! ## Wire Format
+//!
+//! ```json
+//! {
+//!   "template": "usr",
+//!   "data": { "first_name": "joe", "status": "active" }
+//! }
+//! ```
+//!
+//! `template` names the data object (maps to a NamasteCore implementor).
+//! `data` carries key/value pairs in user-facing field names — the template
+//! maps these to actual schema names. Callers never specify primary keys on
+//! writes; the template generates a GUID and returns it in the reply.
+//!
+//! ## Calling Agents
+//! - `brokers::r_broker` — parsed from message body on fetch events
+//! - `brokers::w_broker` — parsed from message body on write/update/delete events
+//!
+//! **Author:** mks
+//! **Version:** 1.0
+//!
+//! ## History
+//! * `2026-04-05` - mks - original coding
+
+use std::collections::HashMap;
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+/// The JSON body of every BEDS broker message.
+///
+/// # Examples
+///
+/// ```
+/// use rustybeds::brokers::payload::BrokerPayload;
+///
+/// let json = r#"{"template":"usr","data":{"first_name":"joe"}}"#;
+/// let payload: BrokerPayload = serde_json::from_str(json).unwrap();
+/// assert_eq!(payload.template, "usr");
+/// assert!(payload.data.contains_key("first_name"));
+/// ```
+///
+/// Both read and write brokers parse this struct from the raw AMQP delivery
+/// bytes. The operation type is carried in the AMQP `type` message property —
+/// this struct carries the object identity and data payload.
+///
+/// # History
+///
+/// * `2026-04-05` - mks - original coding
+#[derive(Debug, Deserialize, Serialize)]
+pub struct BrokerPayload {
+    /// Template identifier — names the NamasteCore implementor to dispatch to.
+    /// Matches the TLA convention from the template file (e.g. `"usr"`, `"pst"`).
+    pub template: String,
+
+    /// Key/value data pairs in user-facing field names.
+    /// For writes: the record to store (pkey excluded — generated by template).
+    /// For reads: query discriminants (field → value to match).
+    /// For deletes: discriminants identifying the record(s) to remove.
+    #[serde(default)]
+    pub data: HashMap<String, Value>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn deserializes_full_payload() {
+        let json = r#"{"template":"usr","data":{"first_name":"joe","status":"active"}}"#;
+        let payload: BrokerPayload = serde_json::from_str(json).unwrap();
+        assert_eq!(payload.template, "usr");
+        assert_eq!(payload.data.len(), 2);
+        assert_eq!(payload.data["first_name"], "joe");
+        assert_eq!(payload.data["status"], "active");
+    }
+
+    #[test]
+    fn deserializes_without_data_field() {
+        // data is optional — fetch by template name alone is valid
+        let json = r#"{"template":"usr"}"#;
+        let payload: BrokerPayload = serde_json::from_str(json).unwrap();
+        assert_eq!(payload.template, "usr");
+        assert!(payload.data.is_empty());
+    }
+
+    #[test]
+    fn serializes_round_trip() {
+        let json = r#"{"template":"pst","data":{"title":"hello"}}"#;
+        let payload: BrokerPayload = serde_json::from_str(json).unwrap();
+        let serialized = serde_json::to_string(&payload).unwrap();
+        let round_trip: BrokerPayload = serde_json::from_str(&serialized).unwrap();
+        assert_eq!(round_trip.template, payload.template);
+        assert_eq!(round_trip.data["title"], payload.data["title"]);
+    }
+
+    #[test]
+    fn rejects_missing_template() {
+        let json = r#"{"data":{"first_name":"joe"}}"#;
+        let result: Result<BrokerPayload, _> = serde_json::from_str(json);
+        assert!(result.is_err());
+    }
+}