Review pass:
- Drop `Pick<>` type aliases on `Usage.totalInput` / `Usage.totalOutput`
— the helpers can take `Usage` directly since every field is optional.
- Collapse Bedrock's nested `subtractTokens(subtractTokens(...))` into a
single subtraction against the summed cache subtotals.
- Drop arithmetic-walkthrough comments in test fixtures (the raw
fixture values are right next to the expected outputs).
- Generalize the comment on `mapUsage` in `openai-chat.ts` so the
rationale outlives the PR reference.
Kit Langton
478f3ae50c