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/it_task.rs

@@ -19,9 +19,10 @@
 //!
 //! Coverage rules (see the family test charter):
 //!   1. no mocks — every call goes through the real facade into the real
-//!      oaktask/oaknode/oakundo/oakcodec module crates (the only stubs are
-//!      the host-provided `oakcore_*` symbols in `tests/common`,
-//!      the same mechanism the other family tests use);
+//!      oaktask/oaknode/oakundo/oakcodec module crates (the
+//!      `oakcore_audioparams_*` accessors the facade reads through are its
+//!      own in-dylib implementations, re-exported by `tests/common`, the
+//!      same mechanism the other family tests use);
 //!   2. every one of the 27 `oakengine_task_*` / `oakengine_cli_task_*`
 //!      exports is exercised on a legal path with the result asserted;
 //!   3. legal-input matrix (compression flags, url counts, indices, buffer