Improve bridge-inner adapters

[ducky64]

Instead of having the adapter on all ports of the link type to account for bridge polarity (eg, as_digital_source appears on VoltageBase instead of only VoltageSource, because it's compatible on a bridged VoltageSink), move those back into the specific port type, and add a .inner() on bridge-capable ports which returns the inner-facing (bridged) port. This would generate and cache the bridge.

It would be an error to call inner() on a direct export.