fix(transport): multiple devices on same bus use correct slave ID (#235)

Multiple devices sharing the same serial port were using the wrong slave ID
because TransportManager pooled transports by connection key (excluding slave ID)
and modbus-serial client had a static slave ID set once at creation time.

Solution: SlaveTransport wrapper
- Created SlaveTransport class that binds a slave ID to a shared client
- TransportManager now pools modbus-serial clients (not Transport instances)
- Each device gets its own SlaveTransport with its own slave ID
- Slave ID is set before each operation within a mutex-protected context
- Multiple devices on same bus share the same client and mutex

Changes:
- Add SlaveTransport class implementing Transport interface
- Modify TransportManager to pool clients by physical connection
- SlaveTransport sets slave ID before each operation
- Shared mutex ensures serialized access across all slaves
- Absorb retry logic into SlaveTransport
- Add comprehensive documentation for shared resource ownership
- Enable e2e test for multiple devices on same bus

Tests:
- Add comprehensive SlaveTransport tests (21 tests)
- Update TransportManager tests for new architecture (17 tests)
- All 1534 tests pass
- E2E test validates multiple devices work correctly

Closes #171

Author: groupsky

packages/transport/src/index.ts

@@ -12,3 +12,4 @@ export { createModbusTransport } from './create-modbus-transport.js'
 export { withRetry, MAX_RETRIES, RETRY_DELAY_MS, type RetryLogger } from './retry.js'
 export { TransportManager, type TransportStats } from './manager.js'
 export { MutexTransport } from './mutex-transport.js'
+export { SlaveTransport } from './slave-transport.js'