docs(vulkan): isolate backend modernization RFC

[marksverdhai]

Summary

• isolate RFC-0001 from feat(vulkan): native backend inference ops and INT8/INT4 quantization #2 as a documentation-only decision surface
• preserve the RFC blob byte-for-byte from feat(vulkan): native backend inference ops and INT8/INT4 quantization #2
• keep implementation slices independent of the architecture decision

Gate

Draft pending an explicit decision: retain the C++ Vulkan backend as a legacy/mobile fallback, or follow the RFC's clean-replacement plan.

Verification

• one-file scope
• combined with the code stack, the content tree exactly matches feat(vulkan): native backend inference ops and INT8/INT4 quantization #2 head cada6816
• independent static review: approve

[hai-pilgrim]

Backend modernization RFC documentation. Clean separation of design rationale from implementation.

Acting-Agent: Pilgrim