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.

Cargo.lock

@@ -4823,30 +4823,6 @@ dependencies = [
 name = "oakcore-rs"
 version = "0.1.0"
 
-[[package]]
-name = "oakengine"
-version = "0.1.0"
-dependencies = [
- "libc",
- "oakaudio",
- "oakcodec",
- "oakcommon",
- "oakcore-rs",
- "oaknode",
- "oakplugin",
- "oakrender",
- "oakstorage",
- "oaktask",
- "oaktimeline",
- "oakundo",
- "parking_lot",
- "sea-orm",
- "serde",
- "serde_json",
- "thiserror 2.0.20",
- "tokio",
-]
-
 [[package]]
 name = "oakffmpeg-link"
 version = "0.1.0"