fix(dsl): expose static scalar values in constraints

[Zhendong404]

Summary

• expose compile-time scalar constant values to TileLang constraint proxies via param.value
• thread scalar values through ExpandTileOp operand-spec serialization, Python expand_helper context attrs, and constraint evaluation views
• add regression coverage for descriptor selection and pto.select_kernel(...) scalar-value constraints

Validation

• python3 -m pytest tilelang-dsl/tests/test_tilelang_dsl_v1.py -k 'test_select_descriptor_propagates_static_scalar_values_into_constraints or test_select_kernel_constraints_can_read_scalar_parameter_values or positional_context or materialization_constraints_can_see_specializations'
• build/tools/ptoas/ptoas --version

Closes #397

[Zhendong404]

Included in PR #428