feat(slot): slot entry_count + closed-slot reallocation (dcy-019, BAT-025) + REL-005

dcy-019/issue #18: slot 자원을 진입 횟수 기준으로 고르게 재사용하고 idle 소진 시
closed 슬롯을 직접 재할당한다. 거버넌스는 전부 change-req(신규 REQ 없음).

- 도메인(REQ-002): Slot.entry_count(기본 0) + EXPECTED_SLOT_FIELDS, 상태 머신
  closed→pending_buy 신규 전이(base 8→9, ALLOWED_TRANSITIONS 14→15),
  _apply_transition_fields closed→pending_buy clean-slate + →pending_buy 시 +1
- 영속화(REQ-006): 멱등 마이그레이션 008(entry_count), flat 매핑,
  find_allocatable_slot(idle∪closed, entry_count→updated_at→slot_id) + in-memory 동등
- admission(REQ-007): _admit_buy find_idle_slot→find_allocatable_slot + from_state=slot.status
- 테스트(REQ-003): 전이 카운트/계약 가드 정합 + test_slot_entry_count.py / test_buy_admission_reallocation.py
- 문서(REQ-009): data-model.md / runtime-flows.md 정합
- 기존 daily_symbol_entries(symbol-scoped) 분리·미변경

BAT-025(release-candidate) delivery + change-req REQ-002/003/006/007/009(CHG-20260609-01).
REL-005(confirmed, app-service): BAT-024+BAT-025 묶음. operator-facing run-the-app
스모크로 enter --forced-close-at(BAT-024)·closed 재할당(BAT-025) 실 데몬 검증
(실 PG 마이그레이션 008 round-trip 포함).

검증: 단위 366 passed / 35 skipped(KIS sandbox 1건 deselect — live .env 환경 의존), ruff clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

trex_core/domain/slot/state.py

@@ -1,6 +1,7 @@
-"""Slot state machine: 6 states + 8 allowed transitions (REQ-001, REQ-002).
+"""Slot state machine: 6 states + 9 base allowed transitions (REQ-001, REQ-002).
 
-The transitions match dcy-001 FR-2 and BAT-001 design.
+The transitions match dcy-001 FR-2 and BAT-001 design, extended by dcy-019 with
+`closed -> pending_buy` (closed-slot direct reallocation, BAT-025).
 """
 from __future__ import annotations
 
@@ -24,8 +25,8 @@ class SlotState(str, Enum):
     ERROR = "error"
 
 
-# 8 allowed transitions per dcy-001 FR-2. Any pair not in this set must be
-# rejected by SlotService.
+# 9 base allowed transitions (dcy-001 FR-2 + dcy-019). Any pair not in this set
+# must be rejected by SlotService.
 # Note: `* -> error` is represented as transitions from every non-error state.
 _BASE_TRANSITIONS: frozenset[tuple[SlotState, SlotState]] = frozenset(
     {
@@ -36,6 +37,7 @@ _BASE_TRANSITIONS: frozenset[tuple[SlotState, SlotState]] = frozenset(
         (SlotState.PENDING_SELL, SlotState.CLOSED),   # 5. sell filled
         (SlotState.PENDING_SELL, SlotState.HOLDING),  # 6. sell rejected/rollback
         (SlotState.CLOSED, SlotState.IDLE),           # 7. recovery (with daily_symbol_entries +1)
+        (SlotState.CLOSED, SlotState.PENDING_BUY),    # 9. closed-slot direct reallocation (dcy-019/BAT-025)
     }
 )