feat(#91): IMP-91 u2~u15 multi-mdx regression CI suite + status-board auto-update

- u2~u5: tests/integration/test_multi_mdx_regression.py — MDX_SET=(01..05)
  cached integration runs + status/structural/visual snapshots +
  full_mdx_coverage assertion (9 snapshots populated for 01-05).
- u6~u11: F0 normalize / F1 V4 ranking / F2 slot_payload /
  F3 classifier-only AI / F4 layout / F5 final.html axis per MDX_SET.
- u12: pyproject.toml — pytest-json-report>=1.5 in dev extras.
- u13: .github/workflows/multi-mdx-regression.yml — pytest+artifact CI.
- u14: scripts/update_status_board.py + tests/scripts/test_update_status_board.py
  — idempotent JSON marker updater (3 unit tests pass).
- u15: PHASE-Z-PIPELINE-STATUS-BOARD.md — 30 F0-F5 × mdx01-05 markers
  initialized `?` + workflow wiring.

Stage 4 verify: 59/59 PASS targeted (smoke 6 + updater 3 + integration 50),
386/386 PASS regression umbrella, 0 failures.

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

tests/integration/__snapshots__/slot_payload.json

@@ -0,0 +1,103 @@
+{
+  "_doc": "IMP-#91 u8 — F2 slot_payload axis. Pins step12_slot_payload.json per_zone structural shape (position / template_id / builder / slot_names / list_slot_counts / dict_slot_sub_counts / string_slot_nonempty) for mdx 01-05. Pins SHAPE not literal content — text edits in MDX won't drift this snapshot, but builder swap / slot rename / missing slot / list-cardinality drift will. __empty__ zones have builder=null and zero slots.",
+  "01": [
+    {
+      "position": "top",
+      "template_id": "bim_dx_comparison_table",
+      "builder": "compare_table_2col",
+      "slot_names": ["col_a_label", "col_b_label", "rows", "title"],
+      "list_slot_counts": {"rows": 2},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {"col_a_label": true, "col_b_label": true, "title": true}
+    },
+    {
+      "position": "bottom",
+      "template_id": "construction_bim_three_usage",
+      "builder": "quadrant_flat_slots",
+      "slot_names": ["category_1_body", "category_1_label", "category_2_body", "category_2_label", "category_3_body", "category_3_label", "title"],
+      "list_slot_counts": {"category_1_body": 2, "category_2_body": 2, "category_3_body": 2},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {"category_1_label": true, "category_2_label": true, "category_3_label": true, "title": true}
+    }
+  ],
+  "02": [
+    {
+      "position": "top",
+      "template_id": "construction_goals_three_circle_intersection",
+      "builder": "cycle_intersect_3",
+      "slot_names": ["circle_1_label", "circle_2_label", "circle_3_label", "intersection", "title"],
+      "list_slot_counts": {},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {"circle_1_label": true, "circle_2_label": true, "circle_3_label": true, "intersection": false, "title": true}
+    },
+    {
+      "position": "bottom",
+      "template_id": "__empty__",
+      "builder": null,
+      "slot_names": [],
+      "list_slot_counts": {},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {}
+    }
+  ],
+  "03": [
+    {
+      "position": "left",
+      "template_id": "three_parallel_requirements",
+      "builder": "items_with_role",
+      "slot_names": ["pillars", "title"],
+      "list_slot_counts": {"pillars": 3},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {"title": true}
+    },
+    {
+      "position": "right",
+      "template_id": "process_product_two_way",
+      "builder": "process_product_pair",
+      "slot_names": ["banner_left", "banner_right", "process", "product", "title"],
+      "list_slot_counts": {},
+      "dict_slot_sub_counts": {"process": {"sections": 3}, "product": {"sections": 3}},
+      "string_slot_nonempty": {"banner_left": true, "banner_right": true, "title": true}
+    }
+  ],
+  "04": [
+    {
+      "position": "top",
+      "template_id": "bim_issues_quadrant_four",
+      "builder": "quadrant_flat_slots",
+      "slot_names": ["quadrant_1_body", "quadrant_1_label", "quadrant_2_body", "quadrant_2_label", "quadrant_3_body", "quadrant_3_label", "quadrant_4_body", "quadrant_4_label", "title"],
+      "list_slot_counts": {"quadrant_1_body": 2, "quadrant_2_body": 2, "quadrant_3_body": 2, "quadrant_4_body": 2},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {"quadrant_1_label": true, "quadrant_2_label": true, "quadrant_3_label": true, "quadrant_4_label": true, "title": true}
+    },
+    {
+      "position": "bottom-left",
+      "template_id": "__empty__",
+      "builder": null,
+      "slot_names": [],
+      "list_slot_counts": {},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {}
+    },
+    {
+      "position": "bottom-right",
+      "template_id": "__empty__",
+      "builder": null,
+      "slot_names": [],
+      "list_slot_counts": {},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {}
+    }
+  ],
+  "05": [
+    {
+      "position": "primary",
+      "template_id": "__empty__",
+      "builder": null,
+      "slot_names": [],
+      "list_slot_counts": {},
+      "dict_slot_sub_counts": {},
+      "string_slot_nonempty": {}
+    }
+  ]
+}