feat: add live session diagnostics

[Betafer]

Summary

Add a compact Live session section to the Sender so users can see what the active connection is actually doing instead of inferring it from selected settings.

The Receiver sends one backward-compatible JSON metrics packet per second. Older Senders ignore the new packet type, and a newer Sender continues showing local metrics when connected to an older Receiver.

Reported values

• active transport and local interface
• last measured throughput and latency
• active codec, Receiver renderer and decoder backend
• Sender and Receiver frame rates
• pacing drops, queue/backlog drops and decoder errors

Scope

• one new packet type, 0x39, sent Receiver to Sender
• one small C metrics formatter with focused tests
• no capture, compression, rendering or connection-policy changes
• labels in all five bundled languages

Verification

• rebased onto maint-3.5 at v3.5.1-rc.3
• Receiver make test: 649 checks pass, including seven metrics-format checks
• full Receiver build succeeds
• all 112 Sender XCTest cases pass
• all modified language JSON files validate
• live USB4 session previously verified with hardware HEVC, VideoToolbox, renderer, dual FPS and drop/error counters updating in the Sender

This is a Draft PR so the maintainer can review the protocol field names and dashboard placement independently from the maintenance fixes.