fix: handle unknown union variants without dropping the whole update

[jimmy-phantom]

Problem

When a discriminated-union payload carries a __typename matching no known variant (commonly a server that added a variant the client schema doesn't list yet), union parsing throws. applyMutationEvent caught that throw, logged a warn, and discarded the entire entity update, including unrelated sibling fields, with nothing surfaced. The entity would silently stop updating until a known-variant payload arrived.

Change

• Throw a distinguishable UnknownUnionVariantError at the union-discrimination site.
• In parseData, an unknown variant on an optional field degrades to undefined and the rest of the entity applies; on a non-optional field it rethrows.
• applyMutationEvent surfaces UnknownUnionVariantError via log.error (visible) instead of the silent warn, and still applies no partial state.
• Initial load (applyData) and live updates (applyMutationEvent) behave identically, since the decision lives in the shared parse path.

	optional union	non-optional union
initial load	undefined, rest applies	rejects (error surfaced)
live update	undefined, rest applies	drop update, log.error

Tests

unknown-union-variant.test.ts covers all four cells. Full unit suite green; tsc --noEmit clean.

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