time range filtering

As we know perfetto falls over at some point with large enough traces; can we write a tool that slices our output .ftfs by time range so we can look at parts successfully?

This isn't a _good_ solution but it would be great for usability.

Admittedly it's also a pure Fuschia Format problem as opposed to a magic-trace one but nevertheless there might not be a better place for this to live.

The issue names no source files, tests, or entry points; begin by locating the code that produces .ftfs output and reviewing how Perfetto handles large traces. Done means a usable tool can slice an output .ftfs by a requested time range so the resulting sections can be inspected successfully.

Written by the indexing model from the issue text.