fix: reject non-positive block size multiples

[nisbenz]

Summary

Reject zero and negative values passed to set_block_size_multiple_of.

These values are currently stored without validation and later used by the
SM90 and SM100 kernel-selection heuristics. A zero value can produce
modulo-by-zero or a candidate loop with a zero increment.

This change fails early with a RuntimeError and adds regression coverage for
both scalar and (M, N) inputs.

Testing

• python -m compileall -q tests/test_runtime.py
• git diff --check

[ds-review-bot]

🤖 cc_bot Code Review

LLM API 配额已用尽。

Files reviewed: 0

Issues found: ❌ Review incomplete — LLM response could not be parsed
Error: budget_exceeded

[nisbenz]

can you re do the code review