Fix RTP header wire serialization

[Pie8851]

Summary

• serialize RTP fixed-header bytes explicitly instead of relying on C bitfield layout
• preserve RTP v2, payload type, marker, sequence, timestamp, and SSRC semantics across host architectures

Test plan

• Build the repository integration with tools/ci/native_build_test.sh libs/libpeer
• Build Desktop Tiga with bazel build --config=macos_arm64 //projects/desktop/apps/h106/tiga:h106-tiga
• Verify a macOS Opus packet starts with 80 6f after the change

Made with Cursor

[Pie8851]

Closing this in favor of #269, which provides the smaller targeted fix for the confirmed macOS root cause: Apple headers expose BYTE_ORDER/BIG_ENDIAN/LITTLE_ENDIAN, while the existing selector checks their underscored names.

The explicit RTP serialization here avoids bit-field layout dependence, but it is broader than needed for the currently confirmed issue. #269 preserves the existing layout and corrects the macOS endian-macro mapping.