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>

Cargo.lock

@@ -2247,9 +2247,11 @@ name = "rustybeds"
 version = "0.1.0"
 dependencies = [
  "config",
+ "futures-lite 2.6.1",
  "lapin",
  "mongodb",
  "serde",
+ "serde_json",
  "thiserror 1.0.69",
  "tokio",
  "tracing",