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/lib.rs

@@ -13,7 +13,10 @@
 //! ## History
 //! * `2026-04-02` - mks - original coding
 //! * `2026-04-04` - mks - promoted service modules to pub mod services
+//! * `2026-04-05` - mks - added brokers and core modules
 
+pub mod brokers;
 pub mod config;
+pub mod core;
 pub mod logging;
 pub mod services;