Fix wasm lbug_shared compatibility alias

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Summary

• Add a wasm-only CMake alias so existing lbug_shared consumers resolve to the static lbug target when shared builds are disabled for Emscripten.
• Document why the alias is intentionally static-backed for wasm.

Root cause

The wasm build now intentionally disables BUILD_SHARED_LBUG and builds only the static lbug target, but nightly wasm test helper targets still link through the legacy lbug_shared target name. That produced -llbug_shared during the Emscripten link even though no shared library is built.

Validation

• cmake -B /private/tmp/lbug-cmake-wasm-alias-check -G Ninja -DBUILD_SHELL=OFF -DBUILD_TESTS=OFF -DBUILD_SINGLE_FILE_HEADER=OFF .
• git diff --check -- src/CMakeLists.txt