arch: add build-arch.yml for riscv64 wheels (#1680)

* **Package**: `arch`
* **Version**: `8.0.0`
* **Source**: https://github.com/bashtage/arch
* **Docs**: https://bashtage.github.io/arch/

Compiles arch's two Cython extensions, a bootstrap sampler (`arch/bootstrap/_samplers.pyx`) and a univariate GARCH/ARCH recursion module (`arch/univariate/recursions.pyx`), against numpy's C API via meson-python into a wheel; the runtime econometrics stack (numpy, pandas, scipy, statsmodels) resolves from this registry's own riscv64 builds. Upstream publishes no riscv64 wheel.

Mirrors [upstream's `cron-test.yml`](https://github.com/bashtage/arch/blob/v8.0.0/.github/workflows/cron-test.yml) cibuildwheel job, the only build/test workflow upstream carries.

**Differs from upstream**
- Nothing beyond the riscv64 image.

**Matrix**: cp312/cp313, no cp314/cp314t - statsmodels, a runtime dependency, only has riscv64 wheels on pypi.riseproject.dev for cp312/cp313.

**Testing**
- same as upstream (`arch.test(['--skip-slow', '-n', 'auto'])`)

**License**: OK

Built on cp312; 0 passed, 0 skipped.

Author: luhenry

.github/workflows/build-arch.yml

@@ -0,0 +1,101 @@
+# SPDX-FileCopyrightText: 2026 The RISE Project
+# SPDX-License-Identifier: MIT
+---
+# This workflow is based on: https://github.com/bashtage/arch/blob/v8.0.0/.github/workflows/cron-test.yml
+name: Build arch wheels (riscv64)
+
+on:
+  workflow_dispatch:
+    inputs:
+      version:
+        description: 'arch version to build (git tag without leading v, e.g. 8.0.0)'
+        required: true
+        default: '8.0.0'
+  pull_request:
+    paths:
+      - '.github/workflows/build-arch.yml'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ inputs.version || '8.0.0' }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read  # to fetch code (actions/checkout)
+
+env:
+  ARCH_VERSION: ${{ inputs.version || '8.0.0' }}
+  MANYLINUX_RISCV64_IMAGE: quay.io/pypa/manylinux_2_39_riscv64
+
+jobs:
+  setup:
+    uses: $/.github/workflows/_setup.yml
+
+  build_wheels:
+    needs: [setup]
+    name: Build arch ${{ inputs.version || '8.0.0' }} ${{ matrix.python }}-manylinux_riscv64
+    runs-on: ubuntu-24.04-riscv
+    timeout-minutes: 120
+    strategy:
+      fail-fast: false
+      matrix:
+        # statsmodels, a runtime dependency, only has riscv64 wheels on
+        # pypi.riseproject.dev for cp312/cp313 (CLAUDE.md gotcha 84), so cp314/cp314t
+        # have nothing to resolve at test-install time.
+        python:
+          - "cp312"
+          - "cp313"
+
+    steps:
+      - name: Checkout arch v${{ env.ARCH_VERSION }}
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
+        with:
+          repository: bashtage/arch
+          ref: v${{ env.ARCH_VERSION }}
+          fetch-depth: 0  # setuptools_scm derives the version from git tag history
+          persist-credentials: false
+
+      - name: Build wheels
+        uses: pypa/cibuildwheel@1828c10ab37f080699c7b81cea34097c684a7074  # v4.2.0
+        with:
+          output-dir: wheelhouse/
+          only: ${{ matrix.python }}-manylinux_riscv64
+        env:
+          CIBW_MANYLINUX_RISCV64_IMAGE: ${{ env.MANYLINUX_RISCV64_IMAGE }}
+          # numpy/scipy/pandas/statsmodels are build- and/or run-time requirements;
+          # only our registry has them for riscv64, and only-binary keeps a newer
+          # PyPI release from winning resolution and compiling from sdist.
+          CIBW_ENVIRONMENT: >-
+            PIP_EXTRA_INDEX_URL=https://pypi.riseproject.dev/simple/
+            PIP_ONLY_BINARY=numpy,scipy,pandas,statsmodels
+          # arch v8.0.0 pins pytest<9 itself: its own addopts="--strict" trips
+          # pytest 9's stricter default on a duplicate fixture ID arch never fixed.
+          CIBW_TEST_REQUIRES: pytest<9 pytest-xdist pytest-cov matplotlib numpy scipy pandas seaborn
+          CIBW_TEST_COMMAND: python -c "import arch; arch.test(['--skip-slow','-n','auto'])"
+
+      - name: Check wheel contents
+        run: |
+          python3 - wheelhouse/*.whl <<'EOF'
+          import sys, zipfile
+          names = zipfile.ZipFile(sys.argv[1]).namelist()
+          exts = {n.split("/")[-1].split(".")[0] for n in names if n.endswith(".so")}
+          assert exts == {"_samplers", "recursions"}, exts
+          licences = {n.rsplit("/", 1)[-1] for n in names if ".dist-info/licenses/" in n} - {""}
+          assert licences == {"LICENSE.md"}, licences
+          EOF
+
+      - name: Store wheels
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a  # v7.0.1
+        with:
+          name: arch-${{ env.ARCH_VERSION }}-${{ matrix.python }}-manylinux_riscv64
+          path: ./wheelhouse/*.whl
+          if-no-files-found: error
+
+  publish:
+    name: Publish arch ${{ inputs.version || '8.0.0' }}
+    needs: [setup, build_wheels]
+    permissions:
+      contents: write
+      pull-requests: write
+    uses: $/.github/workflows/_publish-wheel.yml
+    with:
+      artifact-pattern: arch-${{ inputs.version || '8.0.0' }}-*-manylinux_riscv64