Tests: harden load-sensitive web, device, desktop and queue tests seen in the v0.15 batch #263

Follow-up from #237 (PR #257). These tests failed under load during the v0.15 batch. Each also fails on main and was out of #237's scope:

• Web: a ChatView test, "opens and closes the panel without scrolling the timeline while holding a just-sent turn anchored" (the scroll spy was called twice).
• Linux, device tests pinned to 2 CPUs: scrcpy::tests::cancelled_start_removes_the_forward, start_fails_and_removes_the_forward_when_nothing_listens, and the media_socket::tests slow-close tests. They time out at the 10 s wait_until limit.
• Windows desktop (CI, PR feat(mcp): SSH agent clones pinned to the checked address behind the Terminals gate #249): backend::tests::an_unparsable_operation_id_reads_the_same_on_both_transports failed with "SQLite online backup WAL checkpoint remained busy (163/163 pages)".
• Windows, full parallelism: four turn_delivery::queue_tests (a_queued_follow_up_waits_for_the_ended_turns_checkpoint, the two …does_not_delay_another_threads_checkpoint tests, and an_expiry_during_a_taken_capture…) wait 5 s for git checkpoint refs. Each passes alone and at --test-threads=2.

Acceptance

Reproduce each under controlled load, harden the test (condition waits, not fixed sleeps) without weakening a product assertion or deadline, and show failure counts before and after. Follow docs/testing/flaky-tests.md.