feat(count): add QueryCount pagination helpers

Adds CountResult, QueryCount, BuildQueryCountFn, and ReadableTable
alias for Jet pagination total counts, with tests and docs.

Co-authored-by: Cursor <cursoragent@cursor.com>

doc.go

@@ -17,6 +17,7 @@
 //
 // Dialect-neutral type aliases ([Column], [ColumnList]) and helpers for column
 // lists ([NormalCols], [ContainsCol]) and expression building ([ExprValues]).
+// [QueryCount] and [BuildQueryCountFn] support pagination total counts.
 //
 // Partial-update helpers ([ApplyPtr], [ApplyVal]) track changed fields for
 // repository patch logic.