fix: apply the probe-backoff priority penalty to strict-mode ranking

compareAccountsStrict compared raw a.priority - b.priority directly. PR
#429 (landed on main after this branch forked) rewired every OTHER
priority comparison in this file and in strategies/index.ts through
compareAccountPreference, which lets a sustained probe-backoff penalty
(an account refusing for over an hour) outrank a configured priority
number — but because compareAccountsStrict is new code, not an edit of
an existing comparison, neither git nor GitHub's diff view flagged the
mismatch as a merge conflict.

A backed-off account with a numerically better priority than its
competitors would therefore still win strict-mode position 0 on
priority alone, shipping without the fix the rest of the file already
has.

Now compareAccountsStrict calls compareAccountPreference(a, b, now),
matching the sibling sticky comparator's use of the same function. A
new characterization test pins that a penalised account loses to a
healthy lower-priority one, and that priority order is unchanged when
nobody is penalised. All 14 pre-existing strict-mode tests still pass
unmodified — none of them exercise a priority tie, so none depended on
the raw comparison being replaced.