test(tracking): cover the weighted rate with isolated in-memory cases

Move the weighted-rate regression test off the real tracking database, which
it wrote two rows into on every run, and onto an in-memory tracker. Add the
long-tailed pair that pins the weighted rate against the mean of percentages,
the same assertion through get_summary(), and a zero-input group: without the
CASE guard that last one fails on a NULL rate, and no other test covers it.

Name the 4th field of GainSummary::by_command for what it now holds, and state
the rate formula on the CommandStats warning in percentage order, with the zero
guard, so that following it verbatim cannot reintroduce the NULL.

Head the column Total% rather than Rate, keeping the shape of the Avg% it
replaces and naming the total the figure is now taken over.

Drop the hand-written CHANGELOG entry: release-please generates it from the
commit subject, and CONTRIBUTING.md asks for that file to be left alone.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>