codegen: void main() should exit 0, not leak a garbage exit code

[cs01]

Bug

fn main() with no return type (a void main) was compiled to define void @main(...). But main is the process entry point — the OS reads its return register as the exit code. A void function leaves garbage in that register, so a program that should succeed exited nonzero (consistently 10 in testing), e.g.:

fn main() {
    print(42)   // prints 42, then exits 10
}

This is why fixtures had to end in a string print or use fn main(): i32 { ... return 0 } to pass — an int-ending void main failed the harness's exit-code check. Surfaced while adding the compound-bitwise fixture.

Fix

In genFn, force @main to be i32-returning regardless of the Milo signature, and make a bare return inside main emit ret i32 0 (so it matches the forced signature). The existing fallthrough already emits ret i32 0 once the return type is i32. Explicit return <n> exit codes are preserved.

Tests

• tests/fixtures/voidMainExit.milo — void main ending in print(<int>), must exit 0.
• Verified: fn main(): i32 { return 5 } still exits 5; full suite 349 pass / 0 fail.