[DRY/KISS] Consolidate shared test fixtures and infrastructure

[github-actions]

Audit finding

The test suite repeatedly defines local variants of:

• TemporaryDirectory;
• STA-thread runners;
• repository-root/source-file lookup;
• settings coordinator/store setup;
• application identities and assignments;
• fake runtime overlays and notification lists.

The duplicated setup obscures test intent and creates multiple cleanup/timeout implementations.

Target state

Create small shared test utilities and focused scenario builders. Avoid one universal “god fixture”.

Suggested utilities:

• TemporaryTestDirectory;
• StaTest.Run;
• repository/source locator;
• settings test store/coordinator factory;
• assignment/profile builders;
• focused fake/recording ports.

Acceptance criteria

• Duplicate temporary-directory and STA-runner implementations are removed.
• Shared cleanup is reliable after failed assertions.
• Scenario builders expose domain intent rather than implementation details.
• Fakes remain focused per subsystem.
• Test names and arrange/act/assert flow become shorter and clearer.
• Test parallelism and Windows-specific apartment requirements remain correct.