[bug-003] Render references outside the collapsible step wrapper

feat-004's reference_added events were getting grouped into the
PreResponseWrapper alongside other tool chatter, which collapses to
"Completed in N steps" by default. References were invisible until the
user expanded the disclosure - which is exactly backwards: references
ARE the answer when Olava under-synthesises (which it does on
multi-source research queries; see Lawyer Tech Misuse Case test where
Olava emitted only "\n\n" three times alongside 11 web references).

Fix: extend the EventGroup discriminated union with a "reference" kind
that breaks the pre-wrapper just like content does. Reference cards
now render inline at top level, never hidden behind a disclosure.

Verified against the chat that motivated the fix - same 11 references
will now render as cards instead of collapsing into "Completed in 5
steps" wrappers. Synthesis quality (Olava emitting empty content) is a
separate problem addressed by feat-005.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>