fix(strategy): reject a degenerate n in runner before building a Backend

[statxc]

PR kind

• fix
• feat

Summary

matmul.runner.run rejects a degenerate n before touching the GPU, but strategy.runner.run / .compare do not — they construct a Backend (demanding a real GPU) and only then fail deep inside storage:

• n=0 → ZeroDivisionError from the // (n * 8) block-size math
• n<0 → numpy negative dimensions are not allowed

Reproduced on CPU (no GPU needed — the asymmetry is visible in which error you get):

matmul.runner.run(0)       -> ValueError: n must be a positive integer   # correct
strategy.runner.run(0)     -> RuntimeError: requires PyTorch             # Backend built first
strategy.runner.compare(0) -> RuntimeError: requires PyTorch             # Backend built first

Why it regressed (full transparency)

This check landed once in #268 and was lost when that PR was reverted in de2e149 to untangle a conflict with #232. #232's content was re-landed as 4c44b45 (expose validated spectral alpha), but #268 was never re-landed — and its strategy-side test went with it, so nothing caught the drop. Credit for the original fix belongs to #268; this restores the boundary and adds the coverage that would have caught the revert.

Changes

• strategy/runner.py: reject a non-positive / non-integer n in run() and compare() before Backend(...), mirroring matmul.runner.run's existing check (bool is excluded explicitly, since bool subclasses int).
• tests/test_runner_n_parity.py (new): a parity table driving matmul.run, strategy.run and strategy.compare through the same boundary cases, so the two standalone packages can't drift apart here again — plus an n=1 off-by-one guard. A stub Backend that raises on construction proves validation runs before any device work, so it runs on CPU.

Touches only strategy/ and tests/ — no protected paths.

Validation

uv run --extra test python -m pytest tests/test_runner_n_parity.py -q        # 15 passed
uv run --extra test python -m pytest tests/ strategy/tests/ eval/tests/ -q   # 401 passed, 27 skipped

After the fix, strategy.runner.run(0) / .compare(0) raise ValueError: n must be a positive integer, got 0 with no GPU required.

Checklist

• CPU-safe validation passed.
• My commits do not include Co-authored-by footers for coding agents such as Cursor, Codex, Claude, Copilot, or similar tools.
• Not a feat PR — no scorecard required.

[github-actions]

This PR currently has merge conflicts with the base branch. Please resolve them within 12 hours or the bot will close the PR automatically.

[github-actions]

Closing this PR because the reported merge conflict was not resolved within 12 hours of the bot reminder.