Add an RFC for InternalPort.

[wanda-phi]

Rendered.

[whitequark]

We have discussed this RFC on the 2025-06-09 weekly meeting. The disposition was to merge, resolving the unresolved questions as follows:

• Using InternalPort with a DDRBuffer in simulation causes the buffer to elaborate to an unspecified glitch-free model driving the port triple.
• Using InternalPort with a DDRBuffer in synthesis is forbidden. If a need to abstract over unknown implementations of DDR buffer exists for a component, this should be done by using io.DDRBuffer.signature in the component interface rather than using InternalPort or similar objects.