scan_next: tag re-centered Plan B plans in the log

scan_next: tag re-centered Plan B plans in the log

Add a "pivot" marker to the plan: line for destinations of a re-centered Plan B
(prefer-canonical-src pivoted the star onto a higher-ref whole-extent duplicate),
so re-centered plans are greppable when validating the pivot against a run.
DstPlan carries an m_pivoted flag set on each pivoted destination at build time;
the plan: line prints " pivot" when it is set.  Document the tag in running.md.

Assisted-by: Claude-Code:claude-opus-4-8