Function reject_unsupported_sync_aggregate 

pub fn reject_unsupported_sync_aggregate(ast: &Ast) -> Result<(), BuildError>

Reject a relationship sum/avg aggregate anywhere in ast’s related tree — the precomputed-sync path (synthetic aggregate base tables, AGGREGATE-SYNC-DESIGN.md §3) is count-only: it ships the reduce’s (group…, count) row as a base table and has no wire encoding for a sum/avg value or its int-vs-real typing. A relationship sum/avg still works for a direct engine read (the generic scalar projection); it is only unsupported for sync materialization, so registration surfaces it as a [BuildError] rather than silently minting a count-shaped synthetic table. Call this at each sync-registration entry (the replica’s register_shared_query, the room’s materialize) before any of table_tree / agg_table_schemas / NormalizeFold walk the tree — that ordering is what makes the Sum/Avg arm of hash_agg_subquery unreachable.