#676 Tests: split the state-layout move flush crash matrix into one test per flush

Tests: split the state-layout move flush crash matrix into one test per flush. Worker delivery rebased onto main; see the commit messages.

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Summary by cubic

Splits the state-layout move flush crash/recovery matrix into one test per flush so no single test hits the per-test deadline.

• The exact flush-order probe is now its own test, with the flush sites written out from the design rather than measured from the run.
• Each of the eleven crash recovery cases runs in its own test; the slowest takes 1.15 s instead of the 6.79 s the combined matrix took.
• Adds an Unreleased changelog entry.