Parallel-exec hardening: stage-exec-test (from-0 / chaintip / resume-nonchaintip) fails in both serial and parallel modes

## Symptom

On the `EXEC3_PARALLEL` test matrix (#21017), the `QA - Stage Exec Smoke Test` workflow's `stage-exec-test` jobs fail in **both** modes:

- `stage-exec-test (from-0, serial)` ❌ and `(from-0, parallel)` ❌
- `stage-exec-test (chaintip, serial)` ❌ and `(chaintip, parallel)` ❌
- `stage-exec-test (resume-nonchaintip, serial)` ❌ and `(resume-nonchaintip, parallel)` ❌

Because these fail on the serial path too, the root cause is **not** an exec3-parallel race — it's a separate concern (re-exec-from-0 / chaintip sync behavior). It was explicitly scoped out of #21088 (which is the parallel-commitment correctness fix).

## What to do

- Triage the `stage-exec-test` failures starting from the serial-mode logs (since serial failing rules out the parallel-specific paths).
- Identify whether this is a pre-existing failure on `main` or a regression introduced by a recent change.
- Fix or, if it's a known-flaky/infra issue, document and stabilize.

Related: #21017, #21088