Squash-rebase of the collapse-idle-time feature onto SharedHistoryBuffer's
timeline performance rewrite (window virtualization, grouped-event-buffer,
sticky-canvas scroll compositing, imperative panel-shift transforms).

The collapse feature is layered onto SHB's architecture in the
timeline-graph/ directory layout:
- X-axis time->pixel projection flows through the collapse-aware
  TimelineScale/Viewport (projectX); SHB's Y-axis row virtualization and
  imperative active-panel transform are preserved. The two axes are
  orthogonal so they compose cleanly.
- validTimeToDate routes its parse through SHB's cachedParseJSON, keeping the
  session parse cache inside the feature's abstraction.

Reconciliations where the two approaches were mutually exclusive:
- Dropped the per-row position cache: it keyed on (width,y,len) and would
  serve stale points when collapse toggles; its parseJSON cost is now
  covered by cachedParseJSON.
- Kept SHB's CSS grid background and label-only axis (the axis ticks are no
  longer collapse-aware; collapsed regions are still marked by the overlay).
- group-details-row uses SHB's ResizeObserver/onHeight panel-height path
  instead of the $activeGroupHeight store.

Includes one fix to a pre-existing SHB type error in
svg/timeline-graph-row-rendering.test.ts (literal-narrowed comparison).

Timeline segments are $derived from the streaming event list, so applying
the default collapse on init (mid-stream) made the idle-time zigzag appear
and shift as partial-data gaps resolved during initial load.

- Apply collapseAllSegmentsByDefault() from an $effect gated on
  historyCtx.fetchComplete instead of in onTimelineInit, so the default
  collapse runs once against the final, stable segment set.
- Hide TimelineCollapsedLayer while loading so no zigzag/handles render on
  unstable segments.
- Disable the show/hide idle-time toggle until fetchComplete and show the
  Button's loading spinner in place of its icon while the history streams in.