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>

.github/workflows/multi-mdx-regression.yml

@@ -0,0 +1,71 @@
+name: Multi-MDX Regression (IMP-91)
+
+# IMP-#91 u13 — auto-gate the mdx 01-05 acceptance set on every push to main
+# and on PRs targeting main. Failure of any integration test blocks the
+# commit. JSON report is emitted via pytest-json-report (u12 dep) and
+# uploaded as an artifact for u14/u15 status-board updater consumption.
+#
+# [[feedback_validation_first_for_closed_issues]] — fresh subprocess per CI run.
+# [[feedback_auto_pipeline_first]] — no manual review queue; deterministic gate.
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  multi-mdx-regression:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python 3.11
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+          cache: pip
+
+      - name: Install Chrome and ChromeDriver
+        uses: browser-actions/setup-chrome@v1
+        with:
+          install-chromedriver: true
+
+      - name: Install project (dev extras + selenium)
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -e ".[dev]"
+          python -m pip install "selenium>=4.20"
+
+      - name: Run multi-mdx regression tests
+        run: |
+          python -m pytest -q -m integration \
+            tests/integration/test_multi_mdx_regression.py \
+            --json-report \
+            --json-report-file=imp91-report.json \
+            --json-report-omit keywords streams
+
+      - name: Upload pytest JSON report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: imp91-multi-mdx-report
+          path: imp91-report.json
+          if-no-files-found: warn
+
+      - name: Update status-board markers (IMP-91 u15)
+        if: always()
+        run: |
+          python scripts/update_status_board.py \
+            --report imp91-report.json \
+            --board docs/architecture/PHASE-Z-PIPELINE-STATUS-BOARD.md
+
+      - name: Upload updated status board
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: imp91-status-board
+          path: docs/architecture/PHASE-Z-PIPELINE-STATUS-BOARD.md
+          if-no-files-found: warn