refactor(ksuid): replace BinExpr with top-level expr helpers

Adds StringKSUIDExpr and BinaryKSUIDExpr for use with ExprValues, plus
Expr() methods on both types. Removes ExprStringKSUIDs and BinExpr.

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

doc.go

@@ -16,7 +16,8 @@
 // # Jet column utilities
 //
 // Dialect-neutral type aliases ([Column], [ColumnList]) and helpers for column
-// lists ([NormalCols], [ContainsCol]) and expression building ([ExprValues]).
+// lists ([NormalCols], [ContainsCol]) and expression building ([ExprValues],
+// [StringKSUIDExpr], [BinaryKSUIDExpr]).
 // [QueryCount] and [BuildQueryCountFn] support pagination total counts.
 //
 // Partial-update helpers ([ApplyPtr], [ApplyVal]) track changed fields for
@@ -26,7 +27,8 @@
 //
 // [StringKSUID] and [BinaryKSUID] wrap segmentio/ksuid with storage-specific
 // SQL encoding. Both implement [ApplyInterface] via Equal and IsZero for use
-// with [ApplyInterfacePtr]. Both share identical GraphQL string scalar transit.
+// with [ApplyInterfacePtr]. Expr returns a Jet expression via [StringKSUIDExpr]
+// or [BinaryKSUIDExpr]. Both share identical GraphQL string scalar transit.
 // Choose StringKSUID for text columns (VARCHAR, TEXT); choose BinaryKSUID for
 // binary columns (BINARY(20), BYTEA). [ParseStringKSUIDAny] and
 // [ParseBinaryKSUIDAny] accept flexible input for API boundaries.