refactor(oakengine): absorb oakcore host symbols into the dylib

The 'host-provided' oakcore_audioparams_* runtime imports dated from
the deleted C++ host; the facade is their only caller. The dylib now
defines and exports the six symbols itself (repr(C) AudioParams mirror,
liboakcore-compatible semantics), -Wl,-undefined,dynamic_lookup is gone,
and the Windows DLL undefined-symbol blocker is removed by construction
(Windows CI/packaging stays off until a real toolchain verifies links).

crates/oakengine/src/test_support/mod.rs

@@ -21,8 +21,8 @@
 //! They moved here (`src/test_support/`, pulled in by `src/lib.rs` under
 //! `#[cfg(test)]`) and run as unit tests against `crate::*` instead of
 //! `oakengine::*`. The old `#[path = "common/mod.rs"] mod common;` include
-//! is replaced by the single [`common`] declaration below — the
-//! `oakcore_*` mock symbols it defines may exist only once per binary.
+//! is replaced by the single [`common`] declaration below — its
+//! `oakcore_audioparams_*` re-exports may exist only once per binary.
 //!
 //! The node/timeline/render-graph families (and the graph-op tests that
 //! built fixtures through the deleted handle-based module C ABIs) are