Survive changing networks and deepen the receive buffer

Media died silently whenever a client's source address changed. The relay bound a
peer's endpoint once and refused to move it, and the client stopped offering its
binding token after the first bind, so a Wi-Fi/cellular handover stranded the
session in both directions. Add an authenticated Rebind media frame: the binding
token travels in the clear for peer lookup only, and the AEAD tag over header and
token plus the peer's existing replay window are what authorize the move, so a
captured rebind cannot be replayed to redirect someone else's downlink. The client
rebuilds its UDP socket instead of retrying on one still pinned to a vanished
interface.

Nothing judged the control connection live: pings were sent and pongs ignored, so a
blackholed TCP path went unnoticed for minutes while the UI showed a live session.
Treat any server traffic as liveness and fail the connection when it stops, which
drives the existing reconnect.

The receive jitter buffer had lost its depth floor, so a channel without FEC or
DRED played out with no buffer at all and ordinary reordering became concealment.
Restore a one-frame floor, observe every arrival rather than only accepted ones —
a shallow buffer was rejecting the late arrivals that should have deepened it —
and allow playout to hold a frame so depth can follow a degrading link. A stalled
consumer now sheds the oldest queued packet instead of refusing the live talkspurt.

Add a deterministic network-impairment simulation covering bursty loss, jitter,
reordering, duplication, outages and a stalled consumer, a handover test against a
real relay, a replay test for the rebind path, and a blackholed control connection
driven through a freezable TCP proxy.

docs/architecture.md

@@ -68,6 +68,12 @@ need direct native entry points. These are platform adapters, not a second core.
 - `proto/voicecat.proto` is the control-plane schema.
 - Media uses the fixed header and AEAD construction described in `protocol.md` and
   `security.md`.
+- Media frame types are a versioned contract: `Voice`, `Keepalive`, `UdpBinding`, and `Rebind`.
+  `UdpBinding` establishes a peer's endpoint once, in the clear. `Rebind` moves an established
+  endpoint after the client's source address changes, as on a Wi-Fi/cellular handover; it
+  carries the binding token in the clear for peer lookup only, and authorization comes from the
+  AEAD tag over header and token plus the peer's replay window, so a captured rebind cannot be
+  replayed to redirect someone else's downlink.
 - SQLite is the server's persistent store; schema changes require explicit migrations.
 - Client profiles and TOFU pins are local platform data.
 - The ReplayKit ring layout is separately versioned and frozen.