feat(slots-rpc): available_slots capacity hint 추가 (BAT-037 / REQ-073 AC-6)

slots RPC 응답에 available_slots: int|null 포함.
max_active_slots 미설정 시 null, 설정 시 max(0, M-active).
16 tests passed (AC-6 신규 3개 포함), 423 전체 통과.

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

trex_core/runtime/daemon.py

@@ -274,6 +274,7 @@ class Daemon:
             watchdog_store=self.watchdog,
             watchdog_timeout_sec=self.config.safety_sell.watchdog_timeout_sec,
             ledger_outbox=self.outbox,
+            max_active_slots=self.config.max_active_slots,
         )
         transport.consume_rpc_forever(
             topology.slots_queue(self.config.instance_key), handler, self.stop)