fix(ptodsl): accept max_regs as compatibility argument

[Zhendong404]

Summary

• restore max_regs as an accepted @pto.simt compatibility argument
• validate it with the existing positive-integer resource argument checks
• intentionally discard it before tracing so no pto.simt_max_regs attribute or codegen behavior is restored
• document the compatibility-only behavior

This is a scoped compatibility follow-up to #955 rather than a full revert.

Validation

• python3 -m py_compile ptodsl/ptodsl/_subkernels.py ptodsl/tests/test_jit_compile.py
• git diff --check
• production-code search confirms there is no simt_max_regs field or pto.simt_max_regs emission

The full ptodsl/tests/test_jit_compile.py could not run in the local checkout because the existing build-tree Python environment does not provide mlir.ir.