Decide whether voice.md's two halves should stay co-located

## Summary

`.claude/voice.md` holds two things that change for different reasons. Decide whether that's worth splitting, or close this as the deliberate choice it already is.

## Motivation

Surfaced while doing #268. Fowler's divergent change: the prescriptive rules change when an editorial decision gets made, while the descriptive fingerprint changes when new posts publish and the corpus gets re-measured. Two edit triggers, one file.

The file opens by naming the split itself — "Two halves, kept distinct but co-located" — so this may already be settled. Filing it because the reasoning for co-location isn't written down anywhere, and the next person to notice the smell will re-derive this question from scratch.

## Scope

Arguments for leaving it alone are real and may win outright:

- The `CLAUDE.md` portability rule wants durable conventions reachable from the checkout, and more files means more for a web session to find.
- The two halves genuinely inform each other — the file says so: "When the two overlap they agree; when in doubt, the corpus is the evidence."
- The fingerprint recalibrates rarely, so the divergent-change cost is mostly theoretical.

Whichever way it goes, record the reasoning in the file so the question stays settled.

## Acceptance criteria

- [ ] Decision made: split or keep co-located.
- [ ] Reasoning recorded in `.claude/voice.md`.