chore(crates): retire oakengine facade, drop oakcommon handle module

- crates/oakengine moved to crates/oakengine.bk (excluded from the
  workspace): the frozen C-ABI cdylib had no in-workspace consumers
  left after the direct-rlib migration (M14); git history is the
  authoritative backup.
- oakcommon: remove the CHandle module (no remaining users); config
  store and shared value types are unaffected.

crates/oakcommon/src/lib.rs

@@ -29,7 +29,6 @@ pub mod debug;
 pub mod error;
 pub mod ffmpegutils;
 pub mod filefunctions;
-pub mod handle;
 pub mod miscutils;
 pub mod ocioutils;
 pub mod oiioutils;