Adds the event injection pane

[johnnyt]

Why

The Livebook inspector epic (sui-t36) needs a way to fire events at the
session under inspection: a palette of ready-made events from the loaded
fixtures (statifier ADR-0029 keeps recorded sessions replayable only when
input goes through the ordinary event API), with the payload editable
before send, plus a free-form name+payload escape hatch. Without fixtures
the pane degrades to the free-form field alone.

What

Four pure modules, no Kino reference (the sui-t36.8 assembly bead owns the
widget wrapper, matching StatifierUI.EventLog and StatifierUI.Diagram):

• StatifierUI.EventInjection.Draft - form input to %Statifier.Event{}:
  trims and syntactically validates the event name (never against the
  chart - firing an unmatched event is a legitimate debugging act), and
  round-trips payload text through stdlib JSON + StatifierUI.Value
  (ADR-0005). Blank text means data: :undefined, "null" means nil,
  "{}" means the empty map - preserving the three-way distinction
  Statifier.Event itself insists on.
• StatifierUI.EventInjection.Palette / Entry - fixtures'
  example_events to entries carrying canonical prefill text via
  Value.encode/1; an unencodable sample becomes a diagnostic and is
  skipped rather than taking the palette down.
• StatifierUI.EventInjection - the pane model: build/1 sets
  free_form_only? when there are no entries (the degrade rule as a model
  property rather than a rule sui-t36.8 must remember), and send/2 /
  send_draft/3 are the single call site in the repository that puts an
  event into a session, via Statifier.Session.send_event/2 and nothing
  else - never interpret/2, never send_invoked_event/3.

Notes

• Base choice: main, independent of the open PRs. The slice needs only
  StatifierUI.Fixtures (merged, sui-t36.2) and the statifier dep. Not
  stacked.
• send/2 shadows Kernel.send/2, so the module carries
  import Kernel, except: [send: 2] - narrow, standard, no behavioral
  effect.
• Open questions recorded in the plan, both non-blocking: (1)
  Session.send_event/2 is a cast with no correlation id, so tying an
  injection to its trace.event_dequeued is heuristic - a caller-supplied
  correlation id would be an engine change and therefore an st- bead; (2)
  ADR-0003 fixes one sample payload per event name - a multi-sample palette
  would need an ADR amendment plus a new sui- bead.
• Gate: full mix quality and mix gate.verify green after every phase
  and after rebase - 272/272 tests, 95.5% coverage; Gettext/Sobelow are the
  two permanent not-applicable skips. The known sui-hmm flake did not fire.
• Deferred manual verification (iex/notebook checks: prefill readability,
  live-session firing, moduledoc clarity for sui-t36.8) is recorded in
  docs/plans/260822-sui-t36.6-event-injection-pane.md.

Closes sui-t36.6