feat(engineering): V1-A — Q-001 subsystem-scoped + pillar query integration test

Phase V1-A of the Engineering V1 Completion Plan. Scope was kept tight
per the plan: a single Q-001 shape fix and an integration test that
proves the four pillar queries work end-to-end against one seed graph.

Code change:
  - subsystem_contents() in src/atocore/engineering/queries.py returns
    {subsystem, contains: [{id, type, entity_type, name, status}]} by
    walking inbound part_of edges (the inverse of CONTAINS), filtered
    to active children. `type` matches the catalog spec; `entity_type`
    preserves parity with the rest of this module's response shape.
  - GET /entities/Subsystem/{id}?expand=contains route in routes.py
    matches the Q-001 spec invocation verbatim; 400 on unsupported
    expand, 404 on missing-or-wrong-type.
  - Aliased under /v1.
  - The existing project-wide /engineering/projects/{name}/systems
    route stays as-is for Q-004 (whole-project tree).

V1-A acceptance test (tests/test_v1_a_pillar_queries.py):
  - Q-001 subsystem-scoped: Optics → Primary Mirror + Diverger Lens
  - Q-005 requirements_for: Primary Mirror has its single satisfied req
  - Q-006 orphan_requirements: orphan surfaces, satisfied does not
  - Q-017 evidence_chain: supported claim has the FEA result via
    'supports'; unsupported claim does not
  - Q-009 risky_decisions / Q-011 unsupported_claims also asserted
    against the same seed (Codex P2: don't seed data you don't assert)

Plus targeted tests for the Q-001 helper: missing id, wrong type,
nested child subsystems, inactive-child filtering, real /v1 GET
behavior.

Reviewed by Codex twice: GO WITH CONDITIONS at b575773, GO at 0e83525
after the dual-key emit + Q-009/Q-011 + /v1-behavior + nested/inactive
amends folded in.

Tests: 596 -> 605 (+9). Full local suite green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/atocore/api/routes.py

@@ -2393,6 +2393,41 @@ def api_entity_audit(entity_id: str, limit: int = 100) -> dict:
     return {"entity_id": entity_id, "entries": entries, "count": len(entries)}
 
 
+@router.get("/entities/Subsystem/{subsystem_id}")
+def api_subsystem_contents(subsystem_id: str, expand: str = "contains") -> dict:
+    """Q-001 (subsystem-scoped variant) — the spec-shaped invocation.
+
+    Per ``docs/architecture/engineering-query-catalog.md`` Q-001:
+        ``GET /entities/Subsystem/<id>?expand=contains``
+        → ``{ subsystem, contains: [{ id, type, name, status }] }``
+
+    Distinct from the project-wide tree at
+    ``GET /engineering/projects/{name}/systems`` (Q-004), which stays
+    as-is. V1-A only adds this single shape fix; broader query catalog
+    closure is V1-C.
+
+    ``expand`` is currently restricted to ``"contains"``; other expand
+    facets (Q-007 ``constraints``, Q-008 ``decisions``) land in V1-C.
+    """
+    if expand != "contains":
+        raise HTTPException(
+            status_code=400,
+            detail=(
+                f"unsupported expand={expand!r} for /entities/Subsystem; "
+                "V1-A supports only 'contains'."
+            ),
+        )
+    from atocore.engineering.queries import subsystem_contents
+
+    result = subsystem_contents(subsystem_id)
+    if result is None:
+        raise HTTPException(
+            status_code=404,
+            detail=f"Subsystem not found or not a subsystem: {subsystem_id}",
+        )
+    return result
+
+
 @router.get("/entities/{entity_id}")
 def api_get_entity(entity_id: str) -> dict:
     """Get an entity with its relationships and related entities."""