Implement aggregate materialized view counters

[Osvaldo Andrade (osvaldoandrade)]

Summary

• extend materialized view descriptors and SQL DDL with GROUP BY plus COUNT(*) and SUM(col) aggregate specs
• maintain aggregate MV rows through counter-style AtomicUpdate deltas for inserts, updates, overwrites, deletes, and SQL DML hooks
• add an internal Replica.AtomicUpdateMV path for routed aggregate view counter maintenance
• document aggregate MV semantics in ADR 0003

Validation

• go test ./internal/server -run 'Test(EagerHook_AggregatesCountAndSum|SQLMaterializedViewAggregates)'\n- go test ./internal/sql ./internal/catalog ./internal/server ./pkg/client ./pkg/protocol\n- git diff --check\n- go test ./...\n\nCloses Counters: aggregation in materialized views (SUM/COUNT) #519