Correct CalVer generator smoke contract

[DearKarl]

What changed

• Advanced the active research snapshot and smoke contract to 2026.08.12.2.
• Replaced four accepted incomplete-grid priors with legal row-or-column assignment errors.
• Required complete canonical legality and grid coverage for both Gold and prior across all 256 accepted records.
• Moved incomplete-grid corruption into a mandatory negative rejection fixture outside the corpus manifest.
• Added and sealed the 2026.08.12.2 Evidence Card.

Why

Implementation review identified a contradiction between accepting incomplete-grid priors and requiring 256/256 complete prior grids. The revision preserves the immutable 2026.08.12.1 history while making the generator contract internally consistent.

Validation

• 18/18 data-free tests passed under Python 3.12.
• JSON and coverage validation passed.
• Markdown link scan passed.
• Evidence Card SHA256 seal verified.
• git diff --check passed.