Function codegen_update 

pub fn codegen_update(
    b: &mut ProgramBuilder,
    stmt: &UpdateStatement,
    schema: &[TableSchema],
    _ctx: &CodegenContext,
) -> Result<(), CodegenError>

Generate VDBE bytecode for an UPDATE statement.

Pattern: UPDATE t SET col = ? WHERE rowid = ?

Reads ALL existing columns, replaces changed ones, writes back complete record (no partial patches — this is normative per §10.6).