Problem

Currently, in TableMutationEngine.formatLiteral:

if (value == 'NULL' || value == 'null') {
  return 'NULL';
}

If a user legitimately inputs the text string "NULL" into a string/text column, TableMutationEngine treats it as a database NULL rather than the string literal 'NULL'.

Proposed Solution

• In DataGridStagingBuffer, distinguish between an explicit SQL NULL state (e.g. cell cleared or set to null via context action) and entering the string literal "NULL".
• Update TableMutationEngine to safely emit 'NULL' as a quoted string when entered by the user, and bare NULL when representing a null database value.
• Add unit tests verifying distinct handling of string 'NULL' vs SQL NULL.