fix(config): reject a non-positive tile instead of silently miscomputing

[boskodev790]

Lane: fix/bug — CPU-safe validation only, no GPU scorecard, does not enter the GPU queue.

Summary

Config.__post_init__ validates dtype, vram_fraction and storage, but not tile. gemm.multiply selects the tile edge with cfg.tile or auto_tile(...), so a non-positive tile passes through with a silently wrong result rather than an error:

• tile=0 is falsy, so it quietly falls back to auto — the users explicit request is ignored.
• a negative tile is truthy and flows into _tiles(n, T), whose range(0, n, T) is empty. The tiled (i, j, k) loop then writes no blocks, C is returned untouched (all zeros), and the run still reports a normal GFLOP/s. A wrong answer presented as a successful computation is worse than a crash.

>>> from matmul.config import Config
>>> Config(tile=-5).tile      # accepted today
-5
>>> from matmul.gemm import _tiles
>>> _tiles(1000, -5)          # covers nothing
[]

Change

matmul/config.py — in __post_init__, reject tile that is not None and < 1, mirroring the existing validation. None remains the way to request auto-tiling.

Tests (CPU-only, no GPU)

tests/test_config_tile_validation.py: negative and zero tiles raise ValueError; None and a positive tile are accepted; plus a guard documenting that the rejected values are exactly the ones that make the tiled cover empty.

Verification

uv sync --extra test
uv run python -m strategy.smoke                                     # PASS
uv run --extra test python -m pytest tests/ strategy/tests/ eval/tests/ -q   # 99 passed, 23 skipped
uv run --extra test ruff check matmul/config.py tests/test_config_tile_validation.py   # clean

Touches only matmul/ and tests/ — no protected paths (eval/, docs/, .github/, dashboard/).