EZC: Compiler warnings system

[SchoolyB]

Compiler Warnings

Smart warnings that catch bugs before they happen. All warnings on by default, suppressible with `#suppress(Wxxx)`.

Checklist

Variable warnings

• W2001: Unused variable — declared but never read
• W2002: Unused parameter — function parameter never used
• W2003: Shadowed variable — inner scope redefines outer variable name
• W2004: Unused import — module imported but no function called from it

Control flow warnings

• W3001: Unreachable code — statements after return/break/continue
• W3002: Missing return — non-void function doesn't return on all code paths
• W3003: Empty block — if/for/while with empty body
• W3004: Constant condition — `if true {` or `as_long_as false {`

Type warnings

• W4001: Implicit narrowing — assigning int to i8 without cast
• W4002: Comparison always true/false — e.g. unsigned < 0
• W4003: Division by zero — literal divisor is 0
• W4004: Integer overflow — literal value exceeds type bounds

Suppress mechanism

• Parse `#suppress(Wxxx)` on statements and declarations
• File-level suppression with `#suppress(Wxxx)` at top of file
• `--warn-error` flag to treat all warnings as errors

Dependencies

• Requires type checker (EZC: Type checker implementation #1175) for most warnings
• W3001 (unreachable code) and W3003 (empty block) can be done without type checker