Three timing tests take their two minima interleaved, not in blocks

[tannevaled]

Independent of #103 and #105 — branched from main, touches only two test files.

The flake

TestAThreeComponentJPEGIsCachedOnItsSampleTriple failed once during other
work, reporting 8.9× against a threshold of 3×, and passed three times in a
row alone a minute later. The load average was 59.

Each of three tests in jpegspace_test.go took three draws of the witness, then
three of the subject, and compared the minima. That is enough to fail on a busy
machine: the witness can take all three of its turns in a quiet moment and the
subject all three of its own under a spike, and the ratio then reports the
machine rather than the code.

Measured, not argued

The failure could not be reproduced on demand, so instead of guessing, a
throwaway test computed the ratio both ways on the same work — 40 rounds per
scheme per run, under load:

	max of 40	max of 40	max of 40	over 3×
blocked (today)	1.61	2.27	8.80	1 of 120
interleaved	1.53	1.52	1.61	0 of 120

The 8.80 is the failure, reproduced — and close to the 8.9 that started
this.

The blocked scheme also returned a ratio of 0.50 in the first run, which
says the calibrated subject ran twice as fast as the device witness. That is
impossible for the quantity being measured, and it is the plainest
demonstration that its two minima come from two different moments.

The change

pairedBest(t, witness, subject, rounds) in build_test.go draws the two
alternately and keeps each one's minimum; the three call sites hoist their two
documents and call it. The helper's comment carries the table above, because a
test whose threshold is a ratio needs its reader to know what that ratio's own
spread is.

The fourth ratio assertion in the suite is deliberately left alone.
TestAType3GlyphThatShowsItselfIsRefusedRatherThanBounded compares operation
counts and says in its own comment why that is neither a count of today's
parser nor a duration on this machine. It cannot drift with load.

gofmt, go vet clean; 100.0% of statements; no non-test file changed.

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