Deliver BAT-001: bootstrap trex-core + port slot pure domain

Implement REQ-001~004 (dcy-001): scaffold the trex-core package
(pyproject, trex_core/, tests/), port the slot pure domain into
trex_core.domain.slot with import rewrites, add the EXPECTED_SLOT_FIELDS
contract guard (test_contract), and port the state/service unit tests.

Verification: 56 passed, ruff clean, import sweep shows zero external
deps, public __all__ exposes no infra symbols, contract negative case
reproduced. Align project-structure/runtime-flows baselines to the real
tree. Sync SDLC tracking: REQ-001~004 → Implemented, BAT-001 →
release-candidate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/runtime-flows.md

@@ -70,3 +70,4 @@
 ## Change Log
 
 - 2026-06-03 :: bootstrap-baseline 으로 초기 흐름 baseline 생성(slot 도메인 in-process API 흐름 기준).
+- 2026-06-03 :: BAT-001 이식 완료 반영 — preallocate/transit 흐름이 `trex_core.domain.slot` 실제 코드로 구현됨(REQ-004). 영속화/CLI 흐름은 여전히 미구현(범위 밖).