fix: measure timeline rows so the virtual window tracks real heights

[kevinswiber]

Closes #412.

The timeline virtualizer sized its window, spacers, and reveal math from a fixed 52px row
estimate. Real rows render systematically taller when the master pane is narrow (the .event
meta line wraps — easy to hit since the #411 divider) and shorter under compact density, so the
reveal path's post-scrollIntoView repaint re-anchored rows against wrong spacer heights and
pushed the cursor row out of the viewport ("the cursor gets lost in the stack").

Direction (from the issue's three candidates): measure a representative painted row — the
uniform-estimate window model is kept intact, and the estimate is made true.

• ROW_H = 52 stays as the fallback; a live rowH estimate — the mean measured
  getBoundingClientRect().height of painted rows, unrounded — feeds visibleRange, both
  spacers, and scrollTimelineSelectionIntoView. Guards keep the current estimate when there is
  nothing to measure (empty state, no layout).
• Triggers: a debounced (150ms trailing) ResizeObserver on #timeline covers every
  width-changing signal in one place (divider release, window resize, the narrow media query,
  pane open/close, reading mode, and feat: add h/l keys to resize the inspector split from anywhere #414's h/l keys via the same applySplit write); the
  composition root routes the density toggle explicitly (height-only, no width signal); the first
  laid-out paint seeds the estimate; the reveal re-measures synchronously before seeding scroll
  (the load path measures at full width, then the route opens the split and reflows rows before
  the deep-link reveal runs).
• Estimate changes anchor the reading position from real row geometry (anchoredScrollTop):
  the painted row straddling the viewport top keeps its exact offset, with index-space scaling as
  the no-layout fallback — a density flip no longer teleports the content.

Verification: 9 new vitest specs (642 total green), full just check (2172), and live Chrome
against a 3000+-event store — divider at ~406px with wrapped ~126px rows, 40×j/20×k cursor
stepping stays centered; density flip re-derives 126→100 and keeps the cursor visible; fresh
deep-link load reveals centered.

Shoreline: revision rev:2f669a77 was re-captured as rev:73ed4615 (supersedes) after rebasing
onto #414; Codex reviewer agent:codex-412 accepted with independent gate re-runs; the one
advisory (residual density-flip nudge) was owner-approved and closed.