rtc: deliver each sender only its own RTCP report blocks

[haaspors]

Follow-up to #413. The listener routed an incoming RTCP compound to the sender that owns a reported SSRC, but handed it the whole shared compound (s->cbs.rtcp (s->data, buf, s)), so every sender re-parsed the same buffer and sifted out its own blocks — and saw report blocks and feedback addressed to its siblings.

#413 deferred this because delivering only the relevant blocks changes what the sender callback receives.

Decision: keep the RBuffer contract

The issue asked whether the callback should keep taking an RBuffer or move to a decoded view. I kept RBuffer — it is the least disruptive (the receiver side delivers RBuffer too), and the r_rtcp_buffer_add_* builders make reassembly straightforward.

What

Per target sender, the listener now reassembles a fresh compound containing only what concerns it, and delivers that instead of the shared buffer:

• SR/RR — the reception report blocks naming one of the sender's SSRCs, re-emitted as an RR that preserves the report sender's SSRC. (An SR's sender-info describes the remote and is receiver-relevant, so the sender gets an RR.)
• RTPFB/PSFB — copied to the one sender its media SSRC names.
• XR — aggregates per-source blocks and has no write builder, so it is copied whole to any sender it names (a small over-share limited to co-located senders on the same transport; noted rather than papered over).

A new net/rtp primitive, r_rtcp_buffer_add_packet, appends a verbatim copy of a parsed packet — used for the FB/XR copies (and generally useful for reassembling a subset of a compound).

Buffers are built while the source is mapped, then dispatched after unmapping so a callback is free to map the buffer it receives.

Tests

• rrtcp/add_packet_copies_verbatim — the new builder round-trips a packet byte-for-byte.
• rrtc/sender_receives_only_own_report_blocks — a two-block RR (one for the sender, one for an SSRC it does not send) is delivered as a single-block RR carrying only the sender's block, with the report sender's SSRC preserved.

Existing sender_receives_rtcp / sender_receives_xr / sender_receives_bare_xr still pass under the new delivery. Full suite and ASan tier green; the filtering was verified load-bearing by neutering it (the sender then receives both blocks).

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