Live runs in the run trace view now update incrementally. Instead of
reloading the whole trace on every update, the page fetches only the
rows written since its last read and merges them into the tree it
already has, so large live traces keep updating without re-downloading
everything. Progressive trace loading now always uses this live tail.

Mono-RevId: 8cc2387d2af102abc3a17d23b3ad1e0a2846aca5