PR-Agent flagged that trace child rows rendered an extra stage-hint
cell while the header grid template only had 7 columns. Result: every
expanded child shifted module/message/latency/request rightward, and
the stage chip overlapped its neighbour in the trace timeline.
Add a dedicated 72px "Stage" column to the header grid template and
to every row variant. Standalone leaves and trace heads render an
empty span at the same width; trace children with stage hints render
the chip in that slot. Column edges now match across all row types.
Refs #1138, #1151