Engine v0.30.0

Fixed: no flicker between screens on real infrastructure (#112)

On a deployed console the screens took a moment to change and at times
flickered; the sidebar's search, especially, disappeared and reappeared.

• Nothing pops in after the first paint. Controls that need a script --
  the search, Copy, Live, tab bars, group toggles, the SQL workbench's strip
  and recent rail -- were drawn hidden and revealed by the deferred script, so
  a paint before it drew the page without them. One inline line at the top of
  the head now marks the page before anything is drawn, and they are there
  from the first frame. A stream's #deadletter link opens on Dead letter.
• A page the pointer rests on is fetched before the click (speculation
  rules, prefetch only). Never sign-in or sign-out, the API, the assets, or a
  Preview link -- that one is a billed warehouse query.
• A click is acknowledged: a thin bar at the top when the next page takes
  more than 120 ms.
• The swap fades through: the old page leaves, then the new one arrives,
  never both half-visible at once. The sidebar and the top bar hold still.
• The graph comes with the page instead of a round trip later, so the
  workflow and run pages no longer draw an empty canvas first.

No migration. No configuration changes. If the screens still feel slow on a
deployment, check that the proxy keeps Cache-Control: immutable on
/assets/*?v= -- the console sends it, and a proxy that rewrites it costs a
round trip per asset on every screen.