feat: installer integration: fetch plugin assets from cpp-plugins releases #27

Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840) — this is item ③ of that issue ("Modify the installer to retrieve the assets from the new repository"). Depends on the release pipeline (see Relationships).

The installer lives upstream (utils/install_v2.sh, utils/install.py in WasmEdge/WasmEdge); this issue coordinates the change from this side.

• Define the WasmEdge-release ↔ plugin-release version mapping the installer will use (e.g. a compatibility.json published with each release here, or encoded in the installer).
• Open the upstream PR: plugin asset URLs switch from WasmEdge/WasmEdge releases to WasmEdge/cpp-plugins releases for migrated plugins; unmigrated plugins keep the old path until their migration issue closes.
• Keep backwards compatibility: installing plugins for WasmEdge versions ≤ 0.17.0 must still resolve to the old core-release assets.
• Test: installer CI (or a manual matrix) covers installing at least the zlib plugin from the new location on Linux + macOS.
• Update the installation docs in WasmEdge/docs to mention the new asset source.

🤖 Generated by Claude Fable 5 with Claude Code