feat: migrate wasm_bpf #33

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 wasm_bpf to build, test, and ship from this repository.

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

Migration notes: Linux-only; depends on libbpf; upstream tests exist.

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