[roll] Roll fuchsia [trace] Add Custom Track API to Rust bindings

This introduces a dedicated Track API to the fuchsia_trace Rust library,
wrapping Virtual Threads (VThread) under the hood to allow developers to
easily group related status and state-machine events onto a named process-scoped
track.

This avoids having to hack with async events, prevents track name typos,
and offers safe RAII complete duration macros (track_duration!).

Also includes an integration test to verify in-memory FXT output and
updates the tracing documentation.

Original-Original-Bug: 394080630
Test: fx test rust-trace-test (verifies correct FXT record sequence)

TAG=agy
CONV=c9c1f893-692f-4659-a715-95805038c2c1

Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1609249
Original-Original-Revision: 50548cf097d5760986740f5febbb1ec9cd52b245
GitOrigin-RevId: c03e43e1c745b112cde55ed7a8fc044d4899971b
Change-Id: Ic94910f0b8cae8b232ff599e4d733691b5c0fc4b
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1630547
Cr-Commit-Position: refs/heads/main@{#193642}