Tests: launch fake helpers through a stable script

[emaspa]

Changes

Fixes the post-merge CI failure from #96, run https://github.com/emaspa/openxlr/actions/runs/34709759964.

The actual startup error was Linux ETXTBSY, Text file busy. While a test writes an executable script, another thread can fork and temporarily inherit its writable handle. Exec then refuses the script even after the writing thread closed its own handle.

• Use a build-owned executable launcher and keep each fake helper's body in a separate data file.
• Convert the five test-helper factories to the shared helper. No retries, sleeps or relaxed assertions.
• Preserve the operating system's startup reason in plugin-install diagnostics instead of reducing it to an unspecified failure.
• Document the fake-helper convention.

Verification

• A/B stress: old pattern produced 5,265 ETXTBSY failures in 181,758 executions; the stable launcher produced none in 106,051.
• The old pattern reproduced the real suite failure once in 10 runs. The fixed suite passed 13 consecutive runs, 511 tests each.
• Independent combined build and suite passed after integration with mono support: 0 warnings/errors, 530 tests passed, 3 opt-in desktop tests skipped.
• The launcher passes shellcheck.

No production retry policy, audio behavior, version or release changes.