feat: migrate wasi_nn #31

Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840). Blocked by the pilot migration (see Relationships) — follow the recipe in docs/MIGRATING.md once it lands.

Migrate wasi_nn to build, test, and ship from this repository.

• Enable wasi_nn in the standalone CMake build (its existing WASMEDGE_PLUGIN_* option) against the pinned WasmEdge SDK
• Port tests from test/plugins/wasi_nn and make them pass locally — or add a smoke test if no upstream suite exists
• Add CI following the pilot pattern: .github/workflows/build-wasi_nn.yml + .github/scripts/wasi_nn/
• Add the plugin row to README.md
• After the first release containing wasi_nn: tick it off in the core-removal tracking issue

Migration notes: largest plugin: one build/CI flavor per backend (GGML, PyTorch, TensorFlow Lite, OpenVINO, whisper, …) — split backends into checklist items when starting; PR #3 already prototypes the GGML workflow.

Known defect to fix when enabling the imported helper scripts: utils/wasi-nn/build-wasinn-ubuntu-openvino.sh, utils/wasi-nn/install-openvino.sh, and utils/wasi-nn/install-openvino-genai.sh use [[ ! -v "${VAR}" ]] (7 occurrences) — [[ -v ]] expects a variable name, not its value, so set variables can still be clobbered by defaults. Replace with [[ ! -v VAR ]] or : "${VAR:=default}". Found in review: #44 (comment)

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