Expose asynchronous incoming ICE acceptance and transport cleanup completion

[ziaxzulu]

Add C and C++ APIs to prepare a peer from a retained incoming ICE request, install callbacks, and then accept it. STUN integrity is verified before peer construction. Applications can also approve another authenticated source tuple for an existing peer without replacing its SDP, DTLS identity or channels.

Contribution	Dependency
Stop listeners on their original bind address	Corresponding libjuice endpoint behaviour.
Prepare, accept, reject and attach incoming ICE requests	libjuice pending-request API.
Observe final ICE/DTLS/SCTP transport destruction	Independent lifecycle API; resource-count tests use libjuice statistics.

closeAsync() exposes a shared future or callback; C callers use rtcClosePeerConnectionAsync. Completion follows destruction, including externally retained transport references. Blocking waits wrap asynchronous closure; timeout retains caller ownership. C handles still require deletion after completion.

Rejection and shutdown close prepared peers outside listener locks. Certificate import also runs outside the request mutex. The move-only C++ prepared-peer owner rejects and closes abandoned attempts. In C, a returned peer handle remains caller-owned even when later configuration fails. Existing certificate and ICE configuration APIs are reused, and fingerprint verification remains enabled. Constructor counters are compiled only for tests.

Validation: focused incoming-request and teardown regressions pass, covering reentrant callbacks, scoped ownership, authenticated peer reuse and destruction completion. Coordinated JNI tests exercise real UDP/DTLS/SCTP traffic. Release builds pass with diagnostic exports absent. Incoming API, cleanup contract, source attribution.

This owned aggregate draft keeps upstream as a mirror; independent changes remain candidates for separate external submissions.