Add PING/PONG protocol actions and heartbeats=bounce (RTN23c)

[SimonWoolf]

Problem

Browsers may freeze background tabs (see Chrome's energy-saver freezing). A frozen tab's websocket continues to answer transport-level ping frames on the page's behalf while none of the page's own code can run. Since the server determines liveness from the websocket pong, it never detects the client as gone: presence members are not removed, and so on, even though the session is effectively dead.

Change

• New PING (22) and PONG (23) ProtocolMessage actions (TR2, IDL, protocol.md).
• New heartbeats=bounce value (RTN23b, RTN23c). In this mode the server sends PINGs instead of HEARTBEATs to satisfy maxIdleInterval, and closes the transport if the client does not answer with a PONG. Browser clients should send this.
• RTN23c1/RTN23c2: every client must answer a PING with a PONG on the same transport, echoing the PING's id, regardless of which heartbeats value it sent. This is unconditional so the param is purely the client's request for the server to use this mechanism.
• CD2h mentions the new action.
• UTS: tests for the bounce query param, PONG echoing id, PONG regardless of heartbeats param, and PING resetting the idle timer.
• Also adds the missing ANNOTATION member to the IDL ProtocolMessageAction enum.
• Bumps specification version to 6.2.0.

Connection#ping (RTN13) is deliberately left on HEARTBEAT; dedicated PING/PONG avoid the request/response ambiguity (and hence ping-pong loop risk) that reusing HEARTBEAT for server-initiated liveness checks would introduce.

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