[Kernel] Add inventory guards and clean benchmark layout

[BBuf]

Summary

• move standalone attention benchmark and config-search scripts out of the runtime kernel package
• align registered kernel tests and benchmarks with their operator groups, and remove the zero-reference CuTe benchmark helper
• add CPU-only inventory checks for operator groups, registry targets, JIT sources, and AOT compilation units
• reject conflicting registrations for the same operator/backend while keeping identical re-registration idempotent

Related to #29630.

Verification

• pre-commit run --files <changed files>
• PYTHONPATH=python python3 -m pytest -q test/registered/kernels/test_kernel_inventory.py test/registered/kernels/ops/layernorm/test_fused_op.py test/registered/kernels/ops/layernorm/test_kernels_namespace.py (79 passed)
• strict CI registry collection (1907 registrations collected)
• H200: inventory/registry/namespace smoke (77 passed, 1 skipped, metadata-only CPU check excluded)
• H200: add-constant + Inkling relative projection tests (65 passed)
• H200: Hadamard JIT tests (145 passed)
• H200: all relocated standalone benchmark entry points import and show help successfully

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CI States

Latest PR Test (Base): ✅ Run #30460487196
Latest PR Test (Extra): ✅ Run #30478862179

[gemini-code-assist]

Caution

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