Fix Triton AOT multi-context launches

[voltjia]

Summary

• Cache every Triton AOT module/function pair by the current CUcontext while serializing host launch submission.
• Invalidate stale cached launchers with a manifest schema and reject reloaded artifacts that do not provide the context guard exports.
• Cover fresh and reloaded handles across repeated CUDA context transitions, including CPU-visible launcher contracts.

Root cause

Triton's generated AOT launcher stores one CUmodule and CUfunction in process-global variables. After the first launch initializes those values in one CUDA context, a launch through the same DSO in another context reuses invalid handles and returns CUDA_ERROR_INVALID_HANDLE (400).

Validation

• ruff format --check src/ninetoothed/backends/materializers/triton.py tests/test_built_artifact_reload.py tests/test_triton_aot_materializer.py
• ruff check src/ninetoothed/backends/materializers/triton.py tests/test_built_artifact_reload.py tests/test_triton_aot_materializer.py
• python scripts/check_contributing_style.py
• pytest tests/test_triton_aot_materializer.py -q
• pytest tests/test_built_artifact_reload.py::test_triton_aot_handle_is_reusable_across_cuda_contexts -q
• pytest 'tests/test_aot.py::test_add[True-45327-dtype0-bf16-cuda]' -q

pytest output:

$ pytest tests/test_triton_aot_materializer.py -q
...............                                                          [100%]
15 passed in 1.13s

$ pytest tests/test_built_artifact_reload.py::test_triton_aot_handle_is_reusable_across_cuda_contexts -q
.                                                                        [100%]
1 passed in 8.26s

$ pytest 'tests/test_aot.py::test_add[True-45327-dtype0-bf16-cuda]' -q
.                                                                        [100%]
1 passed in 8.54s

$ pytest
============================= test session starts ==============================
platform linux -- Python 3.10.16, pytest-9.0.2, pluggy-1.6.0
rootdir: /tmp/ninetoothed-context-guard-red.zl9Xdv
configfile: pyproject.toml
collected 353 items

======================= 353 passed in 125.75s (0:02:05) ========================