Make journal concurrency regressions portable across Python versions

[luisleo526]

Journal concurrency regressions depended on two incidental properties of the test environment: Python 3.14's Path.exists() implementation and completion of a filesystem operation within 200 ms. Hosted Python 3.12/3.13 jobs exposed those assumptions.

• Inject the STOP-padding race at the append-open boundary, after the stale size is captured. Assert one injected race, append-only padding, and preservation of the STOP payload.
• Require the no-flock control child to complete successfully while the parent still holds its lock. Keep the 15-second subprocess timeout and real positive contention test; remove the machine-speed threshold.

Only tests and their comments change. Runtime, C++ engine, licensing and trading behavior are unchanged.

Validation: all 59 journal/contention tests pass locally on Python 3.12, 3.13 and 3.14. Independent exact-head Grok review completed before pushing each correction. The full GitHub CI matrix must pass before merge.