test: replace sleeps with barriers and isolate shared test state

[datlechin]

Why

• Unit tests waited on wall time (fixed sleeps, run-loop spins, debounce waits), shared global state behind .serialized, and leaked defaults suites into ~/Library/Preferences.
• The UI suite cut in test: rewrite the test suite, test plans and CI #3412 left 69 flows to prove at the unit level.

Change

• Waits use barriers, an injected clock or a shared waitUntil that fails on timeout. Each new barrier was checked with a mutation.
• Shared state runs under GlobalStateSuites or a trait; EphemeralDefaults replaces ad-hoc suites. 165 XCTest files move to Swift Testing; single-literal case groups become parameterized tests.
• The cut UI flows are covered by 13 new unit tests and 48 existing ones.
• The unit host no longer fetches the real plugin registry or writes AppKit split-view keys to com.TablePro. SwiftLint now flags a UserDefaults = .standard default argument.

Verified

• Unit plan: 24,197 cases, 0 failures; wall time 258 s to 193 s. Preferences file count, com.TablePro keys and the plist mtime are unchanged around a full run.

Not in this PR

• Stacked on test: run plugin and UI-free app tests hostless in packages #3418; rebased onto main once that lands.
• No unit seam without a view refactor, so no test: Compare Sync's license explanation, the tag menu's create item, the unlicensed chart, What's New opening its window.
• TableProTests stays serial: with in-process parallelism on, main-actor work queued past the 60 s limit. Stage 5 measures the other options.