Address Task 3b quality review:
- FormatSize now emits invariant decimal notation so a comma-decimal
locale cannot produce stock text its own invariant parser rejects.
- Validation notices embed a bounded single-line preview instead of the
full cell value.
- Documented GetPlateOptions' all-or-nothing throwing contract.
- STA test helpers run the worker as a background thread with a
diagnostic timeout message; added a de-DE fractional stock round-trip
regression.
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