feat: pilot migration: wasmedge_zlib end-to-end (build, tests, CI)

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Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840). Depends on the standalone build system (see Relationships).

End-to-end pilot using wasmedge_zlib — deliberately the smallest plugin (only external dep: zlib; upstream test suite exists). This issue establishes the patterns every other plugin migration copies, and unblocks the 14 parallel migration issues.

• Build: -DWASMEDGE_PLUGIN_ZLIB=ON produces the plugin shared library against the installed WasmEdge 0.17.0 SDK.
• Tests: port test/plugins/wasmedge_zlib, runnable via ctest; document how a test locates the built plugin (WASMEDGE_PLUGIN_PATH).
• CI: one workflow per plugin — .github/workflows/build-wasmedge_zlib.yml + .github/scripts/wasmedge_zlib/<platform>.sh (pattern prototyped in PR WIP ci: build plugins #3). Trigger on PRs touching the plugin's paths plus the shared build system.
• Decide and document the platform matrix for plugin builds (proposal: start with ubuntu-22.04 x86_64; add manylinux + macOS when the release pipeline lands).
• README: add the plugin table with wasmedge_zlib as the first row (name, status, platforms, maintainer).
• Write docs/MIGRATING.md: the recipe the 14 follow-up issues will follow (enable option → port tests → copy workflow → README row).

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