Guard `InferenceSlicer` against OBB callback crashes when `thread_workers > 1` (#2256)
* fix: serialize OBB inference slicer callbacks
* test: simplify OBB slicer regression test
* refactor: simplify OBB slicer fallback path
* test: make OBB slicer regression deterministic
* fix(slicer): add thread_workers validation and lock for OBB warn flag
* docs(slicer): document OBB fallback, merge order, perf note, dual-use key
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