fix: Disable Foxy separate compilation to avoid Boost.Asio ODR crash

[beekld]

Summary

Linking the C++ client into an app that uses Boost.Asio directly crashed the app under Boost >= 1.91. Foxy's separate-compilation mode (FOXY_FAST_BUILD) baked Asio's out-of-line internals into our library with a layout that diverges from a consumer's header-only Asio, producing an ODR violation. Under ASan this surfaces as a heap-buffer-overflow in the reactor mutex constructor. Force FOXY_FAST_BUILD=OFF so Asio stays header-only everywhere, matching how consumers compile it.

Fixes #590.

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Note

Low Risk
CMake-only build configuration change with no runtime logic changes; primary effect is safer linking behavior for mixed SDK/consumer Asio builds.

Overview
Fixes crashes when apps link this SDK alongside their own Boost.Asio usage (notably Boost ≥ 1.91, issue #590).

The build now forces FOXY_FAST_BUILD=OFF before vendored Foxy is configured, so Foxy no longer enables separate compilation of Asio/Beast or links Foxy’s compiled asio.cpp/beast.cpp into the SDK. That keeps Asio header-only in the SDK artifact, aligned with typical consumer builds and avoiding ODR mismatches (e.g. ASan heap-buffer-overflow in reactor mutex setup).

The CURL networking target also drops the public BOOST_ASIO_SEPARATE_COMPILATION / BOOST_BEAST_SEPARATE_COMPILATION compile definitions that are no longer appropriate for this build mode.

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