fix(base-query): only register in objectByClass when type opts in via @Indexable

[deer]

• Commit 9598b3 over-corrected a bug fix by removing the @Indexable guard on objectByClass entirely, causing every indexed object to be registered in class-membership regardless of whether its class declared any indexable fields. The correct rule is: a type participates in class-membership if it carries @Indexable at the class level or has at least one @Indexable function field.
• objectByClass is now populated after the function-map loops in index(), and removed after the function-map loops in unindex(), so that a thrown exception during function processing leaves objectByClass in a consistent state — the two methods are now symmetric.
• Added three regression tests to IndexCompatibilityTests: plain types (no @Indexable anywhere) are excluded from class-membership; function-field-only types are included; and a failed index() call leaves objectByClass clean.