abs-capture-time extension is gated on an RTCP Sender Report and unconditionally rewritten, breaking verbatim end-to-end latency measurement

This is a follow up to #4325, which was closed after #4364 merged. #4364 only fixed extension-ID negotiation — it didn't change how abs-capture-time (ACT) actually gets forwarded on the downtrack, which is a separate, still-present issue.

In downtrack.go, ACT is only attached to a forwarded packet if SFU has received and validated an RTCP Sender Report for that track https://github.com/livekit/livekit/blob/a1be640b12f47387665b42df6ce3826bcfb96e44/pkg/sfu/downtrack.go#L1099-L1121. Until then, ACT is silently stripped from every forwarded packet. We noticed this because we have an integration test suite with a client that reads ACT to measure real end-to-end latency, and it intermittently fails on real (non-loopback) infrastructure. I opened #4812 with a small opt-in fix.