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>

pyproject.toml

@@ -0,0 +1,27 @@
+[project]
+name = "trex-core"
+version = "0.1.0"
+description = "Shared core for the trex product family; first domain: slot lifecycle (storage-agnostic pure domain)"
+requires-python = ">=3.12"
+dependencies = []
+
+[project.optional-dependencies]
+dev = [
+    "pytest>=8",
+    "ruff>=0.4",
+]
+
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.build_meta"
+
+[tool.setuptools.packages.find]
+where = ["."]
+include = ["trex_core*"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+
+[tool.ruff]
+target-version = "py312"
+line-length = 100