Support RFC 6364 and IS-05 SMPTE 2022-5 FEC SDP

[thangldw]

Summary

This implements the layered FEC approach discussed in #38:

• add structured RFC 6364 SDP support for FEC/UDP, UDP/FEC, FEC-FR, fec-source-flow, fec-repair-flow, and repair-window in sdp/;
• map the IS-05-compatible SMPTE 2022-5 topology (one RTP source flow with one or two repair flows per transport leg) in nmos/sdp*;
• keep RFC-only source IDs, encoding IDs, preference, SS-FSSI/FSSI, repair windows, and media stream IDs explicit in sdp_parameters rather than synthesizing them from IS-05 values;
• resolve receiver fec_mode: auto to the highest configured dimension in the example node.

RFC / NMOS boundary

The generic SDP representation accepts the RFC 6364 ABNF, including optional parameters and empty FSSI element values. It validates numeric ranges and the RFC token/separator rules.

The NMOS layer deliberately handles only the form IS-05 can represent. RFC-valid multi-source groups remain available in the generic sdp/ JSON but are not converted to FEC transport parameters. Ambiguous/lossy cases, such as two repair flows with different destination addresses, are rejected because IS-05 exposes only one fec_destination_ip.

The fec_enabled field addition overlaps with #501. It is included here so this PR remains independently buildable from master; I will drop the duplicate during rebase if #501 merges first.

Tests

• RFC example-style round trip covering optional source tag, preference, SS-FSSI/FSSI, empty FSSI values, and both repair-window units
• malformed ABNF, range, ordering, and separator cases
• exact IS-05 SMPTE 2022-5 example interpretation and text round trip
• 2D generation/interpretation with two repair flows
• unsupported topology, missing media reference, and unrepresentable destination-address cases
• nmos-cpp-node build
• full local suite: 2092 assertions in 164 test cases

Related to #38.