Three e2e tests pass only on retry, and CI is configured not to notice

[adamjohnwright]

Found while auditing for settings that accept a failure quietly, after
--legacy-peer-deps turned out to be doing exactly that (#215).

playwright.config.ts sets retries: 2 in CI. A test that fails and then passes
is reported as flaky and the build goes green. The last Tests run on main
reported four:

Test
e2e/download-feedback.spec.ts:212	"says it is working during the wait"
e2e/back-button.spec.ts:40	"leaves the pathway browser without stepping through tabs first"
e2e/interactor-threshold.spec.ts:437	"says so, and offers the way to it" — fixed in #216
e2e/pathway-browser.spec.ts:171	"the Expression tab renders the Expression Atlas heatmap"

Why this matters more than it looks

A flaky test is not a test that is occasionally wrong; it is a test telling you
something and being silenced. The one I fixed had a real race in it — the badges
are added to the graph and the handler that hides them runs afterwards, so
reading once could catch the moment in between. That is a fact about the
application's timing, discovered only because the retry was looked at.

The other three may be the same kind of thing or may be genuine network
non-determinism against third-party services. They are not mine to guess at.

Not a proposal to remove retries

These suites reach real services over the network, and some non-determinism is
honest. The problem is that flakiness is invisible: it is in the log and nowhere
else, so nobody is accountable for a number that can only grow.

Options, roughly in order of cost:

1. Report it. Surface the flaky list in the job summary so it is seen without
   reading the log.
2. Cap it. Fail the build if more than N tests are flaky, so the number
   cannot grow silently.
3. Fix them. test(interactors): stop racing the handler that hides the badges #216 is one; the pattern there — poll rather than read once —
   probably applies to at least one of the others.

How to see it yourself

gh run view <run-id> --log | grep -iE "flaky|retry #"