docs(core): backfill //! module docs on quic/{io,endpoint,pake}

The last three //!-less modules in the tree (plan §2.5 / §3.2):
  - io:       length-prefixed control-message framing (read_msg/write_msg)
  - endpoint: QUIC endpoint construction + transport tuning + the TOFU
              cert-pinning verifier (PinVerify)
  - pake:     SPAKE2 pairing key exchange

Docs only — no code, type, or wire-format change (cbindgen header byte-identical).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-core/src/quic/endpoint.rs

@@ -1,3 +1,5 @@
+//! Shared QUIC endpoint construction (host + client) and transport tuning — keep-alive and idle
+//! timeout, certificate-fingerprint helpers, and the TOFU cert-pinning verifier (`PinVerify`).
 use std::sync::{Arc, Mutex};
 
 /// Shared QUIC transport tuning for BOTH the host and client endpoints. Keep-alive is the