docs(m181): probe completeness asserts row coverage, not probe count

Greptile round 4, a real arithmetic bug in the prior wording: splitting a
multi-command cell into several probes made a complete set fail its own
count-equals-rows assert (R2 alone expands 1 row into 3 probes). Probes
now carry source-row tags and the assert is over row ids — every row
covered or manifest-declared, every probe tagged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>