Harden semantic boundaries and repair dialogue state

Replace nearest-neighbour personal routing with a frozen class-balanced linear head measured on historical, stratified, cross-validation, holdout, and fresh challenge gates (V-702). Close the four repair handoff holes, preserve nested clarification flows, and route Russian possession statements through structural grammar rather than lexical exceptions (V-573). Owner explicitly requested direct commits to master.

internal/router/stagezero.go

@@ -61,5 +61,10 @@ func StageZeroGrammars(acts ActMatcher) []Grammar {
 	// last overall because it matches on the first word alone: "расскажи про
 	// X" is a world question the model called a fact (Vikunja #498).
 	grammars = append(grammars, NarrativeQueryGrammars()...)
+	// Last because it is deliberately broad over the open remainder of a
+	// grammatical declaration. Every explicit question, command, capture and
+	// narrative request above gets first refusal; this catches the Russian
+	// present-tense possession statement the statistical floors call a query.
+	grammars = append(grammars, PossessionStatementGrammar())
 	return grammars
 }