fix(recall): skip over-budget facts instead of stopping, and never answer a match with nothing (#3688) (#3704)

The max_tokens filter stopped at the first fact that did not fit the
remaining budget, so one long fact evicted every shorter fact ranked
behind it, and a budget no fact fits at all (CJK facts of 100-200 tokens
against max_tokens=80) returned an empty list — which to an agent reads
as "this bank has no such memory".

This is the defect #3221 fixed for the source_facts budget in #3419; that
fix never reached the fact budget one function over. select_facts_within_budget
applies the same rule there — skip, don't stop — and adds the floor that
case needs: if not even the top fact fits, it comes back whole and over
budget rather than clipped, since MemoryFact carries no truncation flag.
max_tokens=0 still means "no facts" (#364).

The response surface is unchanged. A "the budget dropped facts" flag was
considered and dropped: recall's candidate set carries no relevance cutoff
of its own, so on a bank of any size it would be true on nearly every call
and tell a caller nothing. The recall log line and trace still report it.