docs(specs): define the normative popup transport

[Wondertan]

Summary

• define the normative popup transport: endpoint admission, message model, delivery guarantees, navigation and closure, continuity across popup-document replacement, and failure semantics, as a chapter of the protocol specification
• browser protocols cite it instead of restating opener, isolation, and continuity mechanics

Rendered entry points:

• Popup transport specification
• Protocol specification index

Motivation

The popup transport's normative rules were interleaved with implementation detail: carrier wire formats, the port keeper, timeouts, API declarations, and diagnostics. Decoupling them keeps each side tidy, but the main reason is layering. Protocol specifications built on top of the popup transport, starting with the ceremony browser contract, need a stable normative description to reference by requirement ID rather than restating browser mechanics or depending on how one package implements them.

Scope

Normative transport only. The @libid/popup package documentation (API, MessagePort and WebRTC carriers, port keeper, test plan, metrics) travels with the implementation in #25. Carrier wire formats are not an interoperability boundary today and stay with the implementation.