Disambiguate recall spans with session context

[Sahith59]

The final English QA isolated one development control where generic recall wording selected an unrelated session even though the correct source session was present. This change adds complete scoped-session similarity to span reranking while preserving the existing turn-level admission boundary, so 561fabcd and d596882b move to their annotated gold sessions without admitting any new query.

Validation:

• 35 focused retrieval tests pass
• frozen all54 screen passes with the exact prior 49-case admission set, 49/49 selected annotated gold sessions, zero annotated-session loss, and 49/54→54/54 lexical answer recoverability
• final110 composition changes only those two packets versus the paid candidate; the other108 are byte-identical and independent verification passes