test(proxy): make the dangling-marker assertion baseline-relative, not absolute

[amiddavid]

Relates to #192.

What this fixes

#188 added a third instance of the defect #192 documents, and it is mine.

TestAnExhaustedRewindReserveNeitherStampsUnbackedMarkersNorHidesItself asserted snap.StashMissing != 0 as an absolute value. StashMissing is a process-wide counter shared with every test in the binary, and TestStatsPublishesDanglingReplaysFromTheLiveCounter — added in the same PR, declared below it in the same file — deliberately drives it non-zero.

So it passed on the first run and failed on the second:

go test ./proxy/ -run "TestAnExhaustedRewindReserve|TestStatsPublishesDangling" -count=2

--- FAIL: TestAnExhaustedRewindReserveNeitherStampsUnbackedMarkersNorHidesItself
    /stats stash_missing = 1 in a fixture where every refusal left the content
    verbatim; a declined removal is being counted as a dangling marker

The failure message was accidentally accurate about its own confusion: on the second run it was reading a different test's dangling marker as this fixture's.

The fix

Baseline-relative — captured before the request this fixture makes, so the assertion measures what the fixture did rather than what the binary has accumulated.

Worth noting the other assertions in the same file were already written this way (the StashRefusals delta, and the live-counter comparison in the test below). This one was not, which is the whole of the defect — an inconsistency inside one file rather than a missing convention.

Verification

gofmt -l . clean · go vet ./... clean · go test ./... all packages pass · the two tests pass at -count=2.

Revert-verified. Restoring the absolute form fails at -count=2:

/stats stash_missing moved by 1 in a fixture where every refusal left the content verbatim

The first cut of that mutation did not compile — reverting the assertion left missingBefore unused — so it was re-cut with _ = missingBefore to fail as a test rather than as a build error. A mutation that breaks the build is not a caught mutation.

Effect on #192

Its count is accurate again. At -count=2 the proxy package now fails only on TestExtractEconomicsAreExported and TestExpandUnresolvedSeriesRender — the two pre-existing instances that issue describes.

How it was found

Verifying, after #188 merged, which issues its merge should have closed. It closed #187 correctly; this turned up because #192 exists and I had added tests to the package it is about, so the honest check was whether I had made it worse. I had.