Make SelectionCopy's clear-then-write test deterministic

[alexkroman]

Problem

SelectionCopyTests › "a clear before the write isn't mistaken for an empty copy" failed on the v0.1.63 release PR's check run (#241). The same code passed on main.

✘ … SelectionCopyTests.swift:116: Expectation failed: await copy.copySelection(maxCharacters: 100) == "late copy"
✘ … SelectionCopyTests.swift:117: Expectation failed: (pasteboard.string → "late copy") == "user's clipboard"

The test modelled the app's late write with a detached Task that slept 30 ms, racing SelectionCopy's 500 ms wall-clock deadline. On a busy runner the task ran after the deadline (the test took 0.640 s). The copier gave up and restored the user's clipboard, and then the late write overwrote it.

Fix

Test-only. FakeCopyPasteboard gets a lateWrite that it applies the moment the cleared clipboard is first read. The copier therefore always sees the clear on one poll and the write on a later one, with no timer to race. The timeout is raised to 5 s. It only bounds the failure case, since a passing run returns after two polls (~20 ms).

Verification

• swift test --filter SelectionCopy passes three runs in a row.
• Mutation check: changing SelectionCopy to stop at the first change (while seen == before) instead of waiting for a string makes the test fail with 2 issues, so it still guards the behaviour.
• swiftlint --strict and swift-format lint --strict pass.

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