feat(macos): Add the UI test suite

[JeanMertz]

The app's unit tests run inside the app process, which puts three things
out of reach: whether a menu item is enabled, what landed on the
pasteboard, and what survives a terminate and relaunch. Those are
exactly the behaviours that break without anyone noticing, and QA.md
carried them as a manual checklist. This is the half of that checklist a
machine can run.

A UI test drives the app through its accessibility tree from a separate
process, so @testable import JP is unavailable and must not be reached
for. Anything checkable in-process belongs in JPTests, where it costs
milliseconds instead of an app launch.

Three things make the suite fast enough to be worth having. A suite
shares one launched app, because launching costs about four seconds and
the work under test costs milliseconds — so a test leaves the app as it
found it, or the suite is ordered so that what one test leaves is what
the next expects. Waits are on conditions rather than the clock, and not
through waitForExistence, which reports an element about a second
after it appears whatever the element; asking again finds it in under
100ms, and the timeout becomes the price of a failure rather than of a
pass. And every animation goes through one lever the tests turn off,
because XCUITest waits for the app to stop moving before each action it
synthesizes.

Anything the tests need the app to do differently goes through
DebugState, gated on #if DEBUG and off unless an environment
variable says otherwise, so a release build has no way to reach it. The
pasteboard works this way and is enforced: no test may touch the system
pasteboard, a debug build copies wherever JP_DEBUG_PASTEBOARD names,
and ClipboardPolicyTests fails on any spelling of the general one.

just test-app-ui is the CI job and runs everything even after a
failure. While writing a test, run it by name instead. fmt-app and
lint-app arrive here rather than with the app because they name every
Swift directory in the repository, and this is the last one to exist.

Signed-off-by: Jean Mertz git@jeanmertz.com