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.toml

@@ -5,11 +5,13 @@ edition = "2024"
 
 [dependencies]
 serde = { version = "1", features = ["derive"]}
+serde_json = "1"
 config = "0.14"
 thiserror = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"]}
 tracing-journald = "0.3"
 mongodb = { version = "3", features = ["sync"] }
+futures-lite = "2"
 lapin = "2"
 tokio = { version = "1", features = ["full"] }