fix(trace): Preserve compressed gaps between autogroup segments (#124456)

Autogrouping separated spans now preserves the inactive gaps eligible
for compressed timelines. Previously, five 100ms spans spaced 225ms
apart lost all four compressible gaps after grouping, even though the
waterfall still displayed separate bars.

Gap detection now uses rendered sibling and parent group segments with
per-segment buffers. The manager also refreshes segment widths and
positions when compression changes after render, keeping grouped bars
aligned during timeline updates and resizing.

Fixes EXP-1201