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

Per docs/plans/engineering-v1-completion-plan.md Phase V1-A. Scope is
deliberately tight: a single shape fix on Q-001 and an integration test
that proves the four pillar queries (Q-001 subsystem-scoped, Q-005,
Q-006, Q-017) work end-to-end against one seed graph on
p05-interferometer.

The shape fix:
  - New subsystem_contents() in src/atocore/engineering/queries.py.
    Returns {subsystem, contains: [{id, entity_type, name, status}]}
    by walking inbound part_of edges (the inverse of CONTAINS), filtered
    to active children.
  - New route GET /entities/Subsystem/{subsystem_id}?expand=contains
    matching the spec invocation in
    docs/architecture/engineering-query-catalog.md Q-001 verbatim.
    400 on unsupported expand value, 404 on missing/wrong-type id.
  - Aliased under /v1.

The existing project-wide /engineering/projects/{name}/systems route
stays as-is for Q-004 (full project tree). The two queries answer
different operator questions and both belong in V1.

V1-A acceptance test (test_v1a_pillar_queries_run_end_to_end_against_
single_seed):
  - Q-001 subsystem-scoped: Optics returns Primary Mirror + Diverger Lens
  - Q-005 requirements_for: Primary Mirror has its single satisfied req
  - Q-006 orphan_requirements: the orphan surfaces, the satisfied does not
  - Q-017 evidence_chain: supported claim has FEA result via 'supports';
    unsupported claim has no 'supports' edge

If this single test fails, V1-A is not done.

Tests: 596 -> 602 (+6). Per the plan: "~4 tests"; the +2 are basic
helper-function negatives (missing id, wrong type) kept separate from
the route and integration tests.

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

src/atocore/main.py

@@ -66,6 +66,8 @@ _V1_PUBLIC_PATHS = {
     "/entities/{entity_id}/invalidate",
     "/entities/{entity_id}/supersede",
     "/entities/{entity_id}/audit",
+    # V1-A: Q-001 subsystem-scoped variant per engineering-query-catalog
+    "/entities/Subsystem/{subsystem_id}",
     "/relationships",
     "/ingest",
     "/ingest/sources",