Task: real-GPU evidence, docs, and release plumbing

[SnowCheetos]

Part of #154.

Resolves map ticket 11 (real-GPU evidence, docs, and release plumbing).

• Real-GPU evidence: full backend suite green on Intel ANV (Arc 140V) and Apple M3 via MoltenVK (local validation only, not a CI lane); lavapipe CI lane green on the MATMUL branch (run 33839537450). Manual hardware commands documented in docs/performance.md per the Build the AMD XDNA NPU backend #75 precedent — no self-hosted runner on a public repo.
• README: the Vulkan row and section claim exactly what tests demonstrate — Static TOSA 1.0 FP32 IDENTITY + MATMUL, direct host/shared/device bindings.
• docs/performance.md: Vulkan section with the warm-path shape, manual commands, the three verified stacks, and a deliberate no-timing-claims note until the XDNA-structured warm-latency measurement lands on the ANV reference box.
• docs/portability.md / docs/architecture.md: claim MATMUL.
• Release policy: the unsafe-exception entry (ash 0.38.0, persistent mapping, VK_ERROR_DEVICE_LOST) was registered with ticket 8's FFI; check-release-policy.py gates 18 packages green.
• Remaining before the map closes: warm-latency measurement (XDNA structure: load once, warm 20, measure 200) on the ANV reference box; FP16 gate (Assign scaffold Target constants for the numerical tier (ticket 5) #174 stays open by design).

[SnowCheetos]

Resolved on vulkan/matmul-9 (PR #187): docs claim exactly what the three verified lanes demonstrate; release-policy unsafe-exception entry registered; warm-latency measurement remains as the map's closing note. Closing.