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/application/messaging_handlers.py

@@ -177,6 +177,7 @@ def make_slots_rpc_handler(
     watchdog_store: Optional[WatchdogStore] = None,
     watchdog_timeout_sec: int = 120,
     ledger_outbox=None,
+    max_active_slots: Optional[int] = None,
 ):
     """Build the RPC handler for `trex.slots.req` (REQ-073/074).
 
@@ -184,9 +185,10 @@ def make_slots_rpc_handler(
       filter: "all"|"inactive"|"pending"|"holding"|"error" (default "all")
       spy=true: watchdog not updated (operator view, REQ-034 CHG-01)
 
-    Response: {slots: [...], degraded: bool, reason: str|null, message: str|null}
-    Each slot row carries slot/core state + hub asset data + watchdog + overlay
-    flags (strategy fields excluded — REQ-074 AC-5).
+    Response: {slots: [...], available_slots: int|null, degraded: bool,
+               reason: str|null, message: str|null}
+    available_slots: capacity hint — null when uncapped, max(0, M-active) when capped
+    (REQ-073 AC-6). Does not replace the admission gate.
     """
 
     def handler(req: RpcRequest) -> RpcResponse:
@@ -248,8 +250,14 @@ def make_slots_rpc_handler(
             reason = reason_code.value if reason_code is not None else None
             message = msg
 
+        if max_active_slots is not None and max_active_slots > 0:
+            available_slots = max(0, max_active_slots - slot_repo.count_active(instance_key))
+        else:
+            available_slots = None
+
         return RpcResponse(payload={
             "slots": slot_rows,
+            "available_slots": available_slots,
             "degraded": degraded,
             "reason": reason,
             "message": message,

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)