Multi-stream: normalise currencies with BigDecimal and a rate feed

Money joined across a rate list is the one place double is wrong. BigDecimal keeps the scale explicit, setScale with a RoundingMode decides the cents, and the total is a reduce rather than a sum over doubles.

• 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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