Five new event types in slice-2's v1 protocol. Forwards-compatible
per slice-2 §3.4: FrameKind's #[serde(other)] Unknown absorbs the
new types in old brains (they log + count + drop). No wire-format
break, no version bump.
New kid on the dispatch: tools.update — brain declares its catalog
on hello so the core's tool registry can validate function_call
events by name (Task 6). The 'function_call'/'function_call_output'
pair is the FOB-boundary dispatch contract the brain's translator
(Task 4) wires to OpenAI Realtime's
response.function_call_arguments.done / conversation.item.create.
TDD: 6 tests fail-red on missing impl, pass-green after this task.
Slice-2's existing protocol tests stay green — the additions are
purely additive.
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