Orca's engine glue wasn't Orca-specific. Move it where the Fusion add-in (and any
later JS-running plugin) can use it too:
- plugins/orca/panel/fins_entry.js -> plugins/shared/engine/fins_entry.js
- plugins/orca/panel/engine_bridge.js -> plugins/shared/engine/bridge.js
- the esbuild step in orca/build.py -> plugins/shared/bundle.py (bundle_engine)
- bridge/entry tests, sensitivity repro and ENGINE-SENSITIVITY.md move with them
No behaviour change: the built Orca plugin and its engine bundle are
byte-identical to main's. The Python V8 loader stays in Orca for now; it moves
to shared when Fusion needs it.
