Add GitHub Actions workflow for PyPI publishing on tag (#61)

Uses PyPI trusted publishing (no API tokens needed).
Triggers on version tags (v*.*.*). setuptools_scm derives
the version from the git tag automatically.

After merging, configure trusted publishing on pypi.org:
  Project: cryptoadvance.spectrum
  → Publishing → Add a new publisher
  → Owner: cryptoadvance, Repo: spectrum
  → Workflow: pypi-publish.yml, Environment: (blank)

Co-authored-by: Nazim <al-munazzim@users.noreply.github.com>

Author: al-munazzim

.github/workflows/pypi-publish.yml

@@ -0,0 +1,40 @@
+name: Publish to PyPI
+
+on:
+  push:
+    tags:
+      - v[0-9]+.[0-9]+.[0-9]+
+      - v[0-9]+.[0-9]+.[0-9]+-*
+
+jobs:
+  publish:
+    name: Build and publish to PyPI
+    runs-on: ubuntu-latest
+    permissions:
+      id-token: write  # Required for trusted publishing
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0  # setuptools_scm needs full history for version
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.10"
+
+      - name: Install build tools
+        run: pip install build
+
+      - name: Build package
+        run: python -m build
+
+      - name: Verify version matches tag
+        run: |
+          TAG=${GITHUB_REF#refs/tags/v}
+          PKG_VERSION=$(python -c "from importlib.metadata import version; print(version('cryptoadvance.spectrum'))" 2>/dev/null || \
+            python -c "import re; print(re.search(r'Version: (.+)', open('src/cryptoadvance_spectrum.egg-info/PKG-INFO').read()).group(1))" 2>/dev/null || \
+            ls dist/*.tar.gz | grep -oP '\d+\.\d+\.\d+')
+          echo "Tag version: $TAG"
+          echo "Package version: $PKG_VERSION"
+
+      - name: Publish to PyPI
+        uses: pypa/gh-action-pypi-publish@release/v1