refactor: separate Ascend TileLang DSL implementations from AOT descriptors.

[zhang-minchao]

Summary

• Move the reusable Ascend TileLang DSL and JIT implementations from the compiler package to xllm/python/kernels_npu/tilelang.
• Keep compiler-owned AOT metadata and lowering logic in thin descriptors under xllm/compiler/tilelang/targets/ascend/aot.
• Initialize the active Python kernel package explicitly after native torch.ops.xllm_ops registration.
• Update AOT cache dependencies, kernel development documentation, and the existing embedded-Python tests.

Motivation

The previous compiler/tilelang/targets/ascend/kernels modules mixed two responsibilities:

1. reusable TileLang program builders and Python JIT launchers;
2. AOT-only registration, specialization, ABI, and code-generation metadata.

This made Python JIT callers depend on the compiler package. Moving the DSL implementation directly under kernels_npu would previously also trigger the NPU semantic package and require the native xLLM operators while the C++ binary was still being built.

This refactor separates those responsibilities and keeps the dependency direction one-way:

Python model runtime -> kernels_npu semantic API -> kernels_npu/tilelang
Ascend AOT compiler -----------------------------> kernels_npu/tilelang

The TileLang leaf package does not depend on the NPU semantic API, torch.ops.xllm_ops, the AOT compiler, or the native xLLM extension.

Key Changes

• xllm/python/kernels_npu/tilelang becomes the owner of TileLang DSL builders, JIT launchers, and implementation-local helpers.
• xllm/compiler/tilelang/targets/ascend/aot owns only AOT descriptors and imports the shared TileLang builders.
• xllm.python.initialize_runtime() selects and initializes the active platform kernel package after native operator registration.
• AOT cache keys include the imported DSL implementation modules so source changes invalidate generated artifacts.
• The obsolete xllm/compiler/tilelang/targets/ascend/kernels package is removed.

Validation

• Pre-commit checks pass: Ruff, codespell, and clang-format.
• The NPU build compiles all 10 migrated Ascend TileLang AOT kernel families.
• AOT registry output matches the pre-refactor registry for all migrated families and specializations.
• mtp_async_input_builder_test passes all 8 tests.
• npu_xllm_ops_test passes all 6 tests.

Reviewer Notes

Please focus on the package dependency direction, the native-operator/runtime initialization order, and the split between reusable DSL implementations and compiler-only AOT descriptors.