Type and route Jingle RTP extensions for BUNDLE

[Ri0n]

Scope

P3 foundation from the native-calls implementation plan. This PR is stacked on jingle/group-negotiation / #94.

Typed RTP extension model

• parse XEP-0293 RTCP feedback into typed description- and payload-level structures
• preserve optional feedback parameters and distinguish omitted parameter values from empty strings
• parse XEP-0293 rtcp-fb-trr-int, including the protocol-defined zero value
• parse XEP-0294 RTP header extensions, senders, parameters and extmap-allow-mixed
• validate XEP-0294 extension IDs using protocol ranges rather than arbitrary unsignedShort values
• parse XEP-0339 sources and source groups into typed SSRC structures
• keep unknown description/payload extension XML opaque and round-trippable
• reject malformed known extension elements instead of silently treating them as unknown capability

Offer/answer semantics

• require accepted RTCP feedback to be an unchanged subset at the same scope
• support the XEP-0293 AVPF trr-int=0 fallback without replacing an explicitly offered interval
• enforce XEP-0294 direction narrowing rules
• keep ordinary header-extension IDs stable
• allow RFC 8285 extended offer IDs to represent alternatives and be remapped to a usable answer ID
• reject duplicate usable header-extension IDs while allowing repeated extended alternative IDs
• keep peer SSRC/source state independent from local source state

Authenticated BUNDLE packet router

• add a private BundleRouter for plaintext RTP/RTCP after successful SRTP/SRTCP authentication
• route RTP MID-first, then by an unambiguous peer/incoming SSRC fallback
• learn MID-authenticated peer SSRCs with a bounded table
• keep peer/incoming and local SSRC namespaces separate so incoming RTP can never route via our local SSRC
• parse one-byte and two-byte RFC 8285 RTP header extensions for negotiated MID
• route RTCP sender/report/media SSRC fields according to their protocol direction
• fail closed on malformed, unsupported or cross-content compound RTCP instead of broadcasting it
• fence queued delivery with a router revision and invalidate learned routes on reconfiguration
• keep route-table replacement transactional

Deliberately not done yet

• no automatic disco advertisement for XEP-0293/XEP-0294/XEP-0339 or BUNDLE
• no claim that the media backend supports every parsed feedback/header extension
• no connection of BundleRouter to the live shared ICE/DTLS/SRTP association yet
• no actual sharing of a live IceConnection by multiple RTP contents yet

The parser, negotiation policy, routing model and runtime capability advertisement remain intentionally separate. The router accepts packets only after authentication; live BUNDLE sharing will be wired only once the P2 membership lifecycle and this P3 demultiplexer are both proven independently.