New timeline panel in the room view (labs)

[langleyd]

Timeline MVVM 3 — new timeline panel in the room view (labs)

Puts the new virtualised timeline into the room view behind the
"Improved timeline scrolling and navigation" labs flag. With the flag off,
nothing changes.

The integration

• A new NewTimelinePanel renders the shared TimelineView +
  RoomTimelineViewModel (from the previous PRs in this series) in place of
  TimelinePanel, switched at a single point in RoomView.
• Events render through the existing EventTile via a small
  LegacyEventTileAdapter, so message appearance is unchanged — tiles will be
  migrated to shared views one by one later.
• The panel manages its own scrolling, read receipts and read marker; the read
  marker uses the new "New" line style from the design system.
• Known gaps, to be addressed in follow-ups: keyboard navigation, scroll-to-bottom-on-send, read receipts aren't displayed yet, and IRC layout falls back to Modern. Also I might try setting last read position and fully read marker on close of the tab.

Keeping the timeline still

The bulk of the fixes here deal with one theme: rows changing height after
they render, shoving the timeline around. Each cause needed its own fix:

• Images — the box reserved for a photo before it downloads was silently
  collapsing to zero (a CSS sizing conflict), so the message grew when the
  photo arrived. The box now genuinely holds the photo's space from first
  paint, and failed images keep the same height instead of collapsing.

• Polls — a poll's question and options are already in the event; only the
  votes need fetching. They now render immediately at full size, and the votes
  fill in without changing the height (previously the tile grew ~180px).

• Reply previews — a new compact mode shows the quoted message immediately
  when it's already loaded, and stands a fixed-height skeleton in while
  fetching one that isn't, so the preview never resizes. Opt-in via an
  EventTile prop; the legacy timeline is untouched.

• Everything else (link previews, late-loading content) — fixes to our
  @tanstack/virtual-core patch so the virtualizer's scroll compensation
  actually holds the view still when a row off-screen changes size:

  • decisions now use the up-to-date scroll position rather than one that can
    be a frame stale during a programmatic scroll;
  • the stay-at-bottom correction retries if the browser clamped the first
    write (the cause of "the timeline stays still while previews
    load and push content off the bottom, so the timline is no longer flush to the bottom");
  • corrections now also apply while scrolling upwards — previously every
    preview or image that finished loading above the viewport shoved the view
    mid-scroll.

  All three are candidates for upstreaming to TanStack Virtual.

Content that grows while it is on screen is deliberately left alone (a
message being edited shouldn't drag the view) — that residual movement is the
same as the legacy timeline and shrinks further as fixed-size designs land
(e.g. the new URL preview style).

Other fixes along the way

• Loading more history could silently stall at the top of the timeline until
  the user jiggled the scroll — requests arriving mid-fetch are now remembered
  and re-run, matching the old ScrollPanel's behaviour.
• Full-width rows (read marker line, date separators) overflowed the
  right-hand edge by the browser's default list indent.