enhancement: narrow fallibility of cast — compile-time string literals are infallible

[sentomk]

Summary

Currently int(s) treats all string sources as fallible, requiring ?: even when s is a compile-time literal that cannot fail. The checker should distinguish compile-time-known strings from runtime inputs.

Changes

1. Narrow fallibility detection in the checker:
   • String literal / compile-time-known string → cast is infallible, no ?: required
   • Runtime input (io::in(), function params, variables) → keep fallible, ?: required
2. Remove ? postfix propagation syntax (int? x = int(s)?)

Examples

string s = "42";
int x = int(s);         // OK — compile-time literal

string age = io::in();
int y = int(age);       // ERROR — runtime input, must use ?:
int z = int(age) ?: 0;  // OK

Testing

• sema pass: literal string to numeric cast without ?:
• sema fail: runtime string to numeric cast without ?:
• sema fail: ? postfix propagation rejected