Test the real matrix interpreter, and add 3.15 pre-release (#259)

Every matrix job was silently testing CPython 3.11: uv sync honors
.python-version (pinned 3.11 by the floor bump) over setup-python's
interpreter, so 'build (3.14)' ran the suite on 3.11.15 (verified in
the PR #288 run logs). UV_PYTHON makes the matrix real. With that
fixed, add 3.15 (final 2026-10-01) as a non-blocking pre-release job so
interpreter breakage surfaces while 2.0 is being built; flip it to
blocking when 3.15 goes final. Locally green today on 3.15.0b3:
pytest, mypy, ruff.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: derek73

.github/workflows/python-package.yml

@@ -20,14 +20,22 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.11", "3.12", "3.13", "3.14"]
+        python-version: ["3.11", "3.12", "3.13", "3.14", "3.15"]
+    # 3.15 is pre-release until 2026-10-01 (#259): surface breakage
+    # without blocking the branch; flip to blocking when it goes final.
+    continue-on-error: ${{ matrix.python-version == '3.15' }}
+    env:
+      # Without this, uv sync honors .python-version (3.11) over the
+      # matrix interpreter and every job silently tests 3.11.
+      UV_PYTHON: ${{ matrix.python-version }}
 
     steps:
     - uses: actions/checkout@v7
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
+        allow-prereleases: true
     - name: Install uv
       uses: astral-sh/setup-uv@v6
       with: