compute_union_filter

Aggregate a set of subscription filters into one covering filter.

Direct port of client/stream/index.ts:#computeUnionFilter.

• Channels are unioned.
• Namespaces: if any subscription omits namespaces (wildcard), the union is unscoped (omits the key). Otherwise, deduplicated union.
• Depth: if any subscription omits depth (unbounded), the union is unbounded (omits the key). Otherwise, take the max. depth=0 is a valid bounded value — never omit when all subscriptions provide it.