add compile-time interface assertions for public types (task c4)

Extend the var _ Interface = (*Concrete)(nil) coverage started in j3 to
cover the remaining public types not yet guarded:

- *file.FdFile → file.File (file/file.go)
- streamrow.Row → globalfilter.Candidate (streamrow/row.go; adds
  globalfilter import — no cycle since globalfilter does not import streamrow)
- *streamrow.RingBuffer → eventstream.Source
  (tui/eventstream/ringbuffer.go; already a type alias for streamrow.RingBuffer)
- *probemanager.Manager → tui/probes.Manager (tui/probes/model.go)
- All 9 generated *types.*Event types → event.Event in the new file
  internal/event/interface_assertions.go (non-generated, lives in the event
  package which already imports types, so no new import cycle)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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