Coverage numbers depend on leftover `fixtures/*/results` from a previous run

[unional]

Found while doing #259 / #260. Not caused by that change — it reproduces on main unmodified.

This suite reports different coverage depending on whether fixtures/*/results already exists:

tree	Statements	Branches	Functions	Lines
cold (after pnpm clean)	96.81	93.42	93.1	96.7
warm (a second pnpm coverage)	97.17	94.73	94.25	96.7

The delta is entirely in src/match.ts. createMatchFunction snapshots the result directory up front, and removeUnchangedFiles only runs when there are prior result files to compare against — so on a cold tree that path and its branches are never taken.

Two consequences:

1. vitest.config.ts carries a comment attributing these two number sets to "platform-conditional paths [that] are not taken on the ubuntu runner". That explanation is wrong. CI reports the lower pair because CI is always a cold checkout, not because of the platform. The comment should be corrected so the next person tuning the thresholds is not misled.
2. The thresholds are effectively pinned to the cold numbers, which is the right choice, but it is accidental rather than stated.

Worth deciding whether removeUnchangedFiles should have a test that exercises it deterministically, which would collapse the two sets into one.