Entering coediting sets the baseline to current content automatically;
'Pin Review Baseline to Now' resets it ('everything so far is settled; show
changes from here'). A coedited doc always has a baseline -> removed the
'no/missing baseline' empty state entirely; the only empty state is a
genuinely empty document. Updated INV-7, PUC-1/7, baseline provider, preview
(marks since checkpoint), failure modes, flags, glossary.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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