Deduplicate _max_smem_per_block_optin across 5 files

[haix716]

Problem

The function _max_smem_per_block_optin is copy-pasted identically across 5 files:

• moonep/prefetch.py
• moonep/grad_reduce.py
• moonep/dispatch_epilogue.py
• moonep/combine_prologue.py
• moonep/combine.py

Recommendation

Move to moonep/_common.py (which already exists and contains shared utilities like inline_ptx, grid_sync, etc.) and import from there.

Impact

Maintainability. If the function needs to change (e.g., new GPU architecture support), it currently requires editing 5 files. Should be 1.