Timeline virtualizer's fixed row-height estimate loses the cursor in narrow panes

[kevinswiber]

The timeline's virtual-window math uses a fixed per-row height estimate (ROW_H = 52 in
src/cli/inspect/web/src/lenses/timeline.ts). Real row heights vary: when the master pane is
narrow — now easy to produce with the resizable divider (#411), or on narrow viewports — the
.event meta line wraps and rows render taller than the estimate. The window math then drifts:
the highlighted cursor row can scroll out of the painted window ("the cursor gets lost in the
stack", found in owner live-testing of #411).

OVERSCAN absorbs small variance and the reveal path's final scrollIntoView corrects exact
offsets, but neither fixes systematic drift when every row is taller than the estimate.

Candidate directions:

• measure a representative painted row and derive the estimate from live layout (re-measure on
  divider release / resize / density toggle);
• or track cumulative measured heights for painted rows (classic dynamic-height virtualization,
  substantially more machinery);
• or clamp the meta line to one non-wrapping ellipsized row so the fixed estimate stays true —
  cheapest, trades away wrapped metadata.

Related: the per-entry scroll-memory work in #411 is unaffected (detail pane, not the list).