rtc: route sender reports to the owning receiver

[haaspors]

Follow-up to #413 — the receive-side mirror of #426.

r_rtc_rtp_listener_handle_rtcp broadcast the whole RTCP compound to every receiver. A Sender Report identifies the remote source it describes in its sender SSRC, which the owning receiver already registers in recv_ssrcmap, so an SR can route to that receiver the same way sender feedback now routes to senders.

What changed

• Each SR is delivered only to the receiver owning its sender SSRC, reassembled into a fresh per-receiver compound, instead of being broadcast.
• Every other packet type (SDES, BYE, and the reception feedback about our outbound streams) is still copied to each receiver, matching the previous behaviour — the issue scoped this to SR / reception statistics, not all receiver RTCP.
• The compound is now mapped once: the per-receiver and per-sender buffers are both built under that single map, then dispatched after unmapping.

Scope note

An RTCP buffer that fails r_rtcp_buffer_map (a malformed compound) is now dropped rather than handed raw to every receiver. Previously each receiver got a buffer its own map would reject anyway, so no useful data is lost. An SR whose sender SSRC no receiver has registered is likewise dropped rather than broadcast — the same routing-by-SSRC semantics as the sender side.

Testing

• New receiver_receives_own_sender_report: an SR for an unowned source is dropped; an SR naming the receiver's source arrives as a lone SR carrying that sender SSRC. Proven load-bearing by neutering the routing (the unowned SR then reaches the receiver).
• Full suite passes; ASan clean on the touched suites.

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