Allocate session caps by occurrence rank after display grouping

When grouping identical displayed passages and limiting results per session, allocate session slots by each occurrence's actual rank. Ordering groups by their best member, then substituting a weak copy from another session, can consume that session's slot ahead of stronger distinct evidence. A simple bounded selection walks ranked authorized occurrences, accepts an occurrence only if its group is still unrepresented and its session has room, and retains other group members as alternate source locations.

This applies to presentation grouping with per-session diversity caps. It does not prescribe entity-max scoring for partitioned ANN or rank fusion. Matching excerpts may have different full-message tails; preserve provenance and avoid claiming content identity. Counts remain local to the examined window, and this cannot recover candidates excluded before grouping.