Add optional multi-version Python test matrix

[leynos]

Summary

Adds an optional multi-version Python test matrix to the template, selectable at render time, modelled on the estate's existing practice:

• femtologging — build-test matrix over 3.12/3.13/3.14 with a 3.15 experimental lane excluded from required checks;
• falcon-correlate — lint job plus a test matrix (3.12, 3.13, 3.14);
• cuprum — typecheck-test legs plus a 3.15a experimental leg with continue-on-error: true.

The template's CI is a single lint-test job, so the change follows cuprum's shape: lint-test is untouched and a new typecheck-test matrix job is added when the toggle is on.

Toggle semantics

New copier.yml boolean multi_python_tests (default false). Help text notes it suits libraries for external consumption; the estate baselines libraries at 3.12 and tests 3.12–3.14. No hidden computed variable was needed — the version list is derived inline in ci.yml.jinja.

Version derivation

The matrix is derived in Jinja from the existing python_version answer: one stable leg per minor version from range(python_version minor, 15) (so a 3.10 baseline yields 3.10–3.14; a 3.14 baseline yields just 3.14), plus a final experimental leg python-version: '3.15', labelled 3.15a per cuprum, with allow-prereleases: true.

Experimental lane is non-gating

The job carries continue-on-error: ${{ matrix.experimental }} and strategy.fail-fast: false; legs are named Typecheck and test (Python <label>) so required-check names stay predictable and the 3.15a leg can be left out of branch protection.

Design notes

• Lint, make audit, Makefile validation, and coverage (plus the CodeScene upload) stay single-legged in lint-test, matching the models. Each matrix leg runs make build, make typecheck, and make test only. Note the 3.13-ish leg re-runs tests that lint-test also runs via the coverage action — cuprum accepts the same duplication.
• With use_rust, every leg sets up the Rust toolchain (shared setup-rust at the template's existing pin) and Swatinem/rust-cache (which keys on matrix values automatically), because make build compiles the PyO3 extension per interpreter — mirroring cuprum and femtologging's per-leg toolchain setup.
• The experimental 3.15 leg may fail to build the PyO3 extension against a prerelease interpreter; that is precisely what the non-gating lane is for.
• Action pins, ${{ "{{" }} … {{ "}}" }} escaping, and indented-tag trailing-whitespace conventions match the existing ci.yml.jinja. The outer toggle uses Jinja whitespace control so the off-state output carries no residue.

Validation

• Rendered all four toggle combinations (matrix × Rust) plus a python_version=3.14 reduced-matrix case with uvx copier copy.
• Matrix-off renders are byte-identical to renders from main for both Rust states (verified with diff).
• Every rendered ci.yml YAML-parses and passes actionlint cleanly.
• Full template contract suite: 38 passed, 2 skipped (act-gated), 3 snapshots passed — including five new parametrized contract cases covering both toggle states, both Rust states, and the reduced 3.14-baseline matrix.
• The developers' guide documents the new behaviour.

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 3557d6fe-a9a1-4147-a868-0150c8419876

📥 Commits

Reviewing files that changed from the base of the PR and between 09a4d88 and beb5022.

📒 Files selected for processing (6)

• copier.yml
• docs/developers-guide.md
• template/.github/workflows/ci.yml.jinja
• tests/helpers/ci_contracts.py
• tests/helpers/tooling_contracts.py
• tests/test_template.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch add-python-matrix-testing

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