fix(vintage): make a preflight failure visible instead of silent

[mspinola]

The cotdata vintage task reported success on 2026-07-31 at 17:00 and nothing appeared in the store on either replica: no new snapshot, no new raw file, no new line in run.log.

The absent run.log line identifies it. The only code between setlocal and the first >> %VINTAGE_LOG% redirect is the mkdir and the preflight block, so a script that reached the first redirect would have appended something even on total failure. It did not, so it exited in preflight. Preflight worked.

The defect is that preflight was unreadable. Its diagnostics went only to stdout, which Task Scheduler discards. Net effect: the check fires, the run does nothing, the task reports success, History shows a clean Task Started / Action completed / Task completed, and the only trace is a Last Run Result code nobody reads because the task looks fine. The message explaining the problem went to the one place that cannot be read.

Changes

• Preflight failures append to vintage-preflight.log beside the script, not in the store. It cannot live in the store, because the likeliest thing preflight catches is a wrong store path, and a log written under a wrong path is equally lost. %~dp0 exists by definition.
• Preflight moved above the mkdir, store-path check first. A half-edited copy would otherwise create a stray directory literally named REPLACE_WITH_STORE_PATH\vintage in the task's working directory, write an entire capture into it, and sync nothing.
• Header names the two things to read when the task claims success but nothing appears, with exit codes decoded: 0x3 wrong store path, 0x2331 venv without the entry points.

No library change. 257 tests still pass.

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