Adds the Livebook inspector assembly

[johnnyt]

Why

The sui-t36 epic's integration piece: the four panes merged over the last
milestone (configuration renderer, event log, event injection, datamodel
explorer) existed only as pure builders with no assembled surface. This
delivers the public entry point - StatifierUI.Kino.inspect/3 - and the demo
notebook that doubles as the milestone's manual acceptance test.

What

• StatifierUI.Trace.Subscriber.attach/3 gains catch_up: true (statifier
  ADR-0049, per the bead's precondition note): the replayed prefix from
  Statifier.Replay.run/1 is folded into the buffer inside the attach call,
  atomically ordered ahead of the live suffix. {:error, :not_recorded}
  falls back to subscribe/2 with a :not_recorded diagnostic - a partial
  stream is labeled Live-only, never presented as whole.
• StatifierUI.Inspector - the pure pane-assembly fold (active
  configuration from the newest trace.macrostep_stable, event log and
  datamodel Markdown, the status header). No Kino dependency; this is where
  the logic and the tests live.
• StatifierUI.Kino - the thin shell, compiled only when the optional
  :kino dependency is present (ADR-0004; a stub raises otherwise). All
  processes go through Kino.start_child/1, so cell re-evaluation
  terminates them and the session's own monitor drops the dead subscriber:
  the clean detach. Injection pane wires fixture-palette buttons and a
  free-form name/payload form to EventInjection.send_draft/3.
• notebooks/inspector.livemd - a walkable checkout-chart demo with
  numbered steps and expected observations per step.

Notes

• Full mix quality green before push (426 tests, 92.3% coverage);
  mix gate.verify attests full scope. Gettext and Sobelow are the two
  permanently-inapplicable skips this project declares.
• Correlation-id question (deferred here from sui-t36.6/t36.7): resolved as
  not needed for this milestone - sent/not-sent feedback plus the live log
  is adequate in a single-user notebook. A caller-supplied correlation id on
  Session.send_event/2 stays a potential engine change for a future
  embedder with exact matching needs; recorded on the bead, nothing filed.
• Replay cost is O(run) and paid in the attaching process; the notebook
  says so where it starts the session.

Closes sui-t36.8.