supercedes #2

high-level:

• Every ${THING}Bridge is a top-level interface extending StableBridgeAPI<>, accessed exclusively through one of:
  • static factory methods (e.g., ${THING}Bridge.create(...)) (no constructors).
  • public AbstractExternal${BRIDGE} implements ${BRIDGE} for things that have expected external implementations.
• The ProxyInternal classes are all effectively private implementation details. Simple implementations are still nested in the interface that they implement, but more complex ones are their own top-level classes.