Analyzer: W0101 warn on expensive multiply/divide

Emits W0101 on \`a * b\`, \`a / b\`, \`a % b\` expressions where both
operands are non-constant (neither is an integer literal). These lower
to calls into the software multiply/divide routines and can blow the
6502 cycle budget. Multiplying by a constant is still silent because
the optimizer can strength-reduce power-of-2 cases to shifts and small
factors to add chains.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/errors/diagnostic.rs

@@ -42,7 +42,6 @@ pub enum ErrorCode {
     E0505, // multiple start declarations
 
     // W01xx: Warnings
-    #[allow(dead_code)]
     W0101, // expensive multiply/divide operation
     W0102, // loop without break or wait_frame
     W0103, // unused variable