Multi-stream: fan a lookup out per key on virtual threads, merge back in key order

One blocking lookup per key is exactly what virtual threads are for. The submission order is not the completion order, so pair each future with its key and sort on the way out - otherwise the report shuffles between runs.

• Multi-stream: a record with a mutable component is a broken Map key
• Multi-stream: an unmodifiable view is not a snapshot of the feed
• Multi-stream: toMap's merge function runs only on a collision
• Multi-stream: nothing runs until the terminal operation
• Multi-stream: indexing the other side costs memory a sorted merge does not
• Multi-stream: UNORDERED is a promise you make, not one you receive

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Published 2026-09-20

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