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/models.py

@@ -39,6 +39,11 @@ class Slot:
     forced_close_at: Optional[datetime] = None  # BAT-014 / REQ-066 (trex-orbit lineage)
     stop_loss_price: Optional[Decimal] = None   # REQ-035 / BAT-010
     take_profit_price: Optional[Decimal] = None  # REQ-035 / BAT-010
+    # dcy-019 / REQ-002 (BAT-025): slot-scoped cumulative entry count. +1 on each
+    # allocation transition (idle/closed → pending_buy); drives admission slot
+    # selection (lowest entry_count first). Distinct from the symbol-scoped
+    # daily_symbol_entries counter (REQ-002 AC-3) — different key/purpose.
+    entry_count: int = 0
     created_at: Optional[datetime] = None
     updated_at: Optional[datetime] = None
 
@@ -64,6 +69,7 @@ EXPECTED_SLOT_FIELDS: frozenset[str] = frozenset({
     "forced_close_at",
     "stop_loss_price",    # REQ-035
     "take_profit_price",  # REQ-035
+    "entry_count",        # dcy-019 / REQ-002 (BAT-025)
     "created_at",
     "updated_at",
 })