feat: add generics support with monomorphization

- Add ekGenericCall to AST for generic function calls (Max<int>)
- Parse generic type arguments in parsePostfix
- Support generic calls in sema with type parameter substitution
- Implement monomorphization in hir_lower:
  - Collect generic function declarations
  - Find all generic call sites
  - Generate specialized versions with mangled names (Max_int)
  - Substitute type parameters with concrete types
- Add generics.bux example

Example:
  func Max<T>(a: T, b: T) -> T {
      if a > b { return a; }
      else { return b; }
  }

  let m: int = Max<int>(10, 20);  // Generates Max_int

examples/generics.bux

@@ -0,0 +1,26 @@
+// Generics - Generic functions and structs
+extern func Std_Io_PrintLine(s: String);
+extern func Std_Io_PrintInt(n: int);
+
+func Max<T>(a: T, b: T) -> T {
+    if a > b {
+        return a;
+    } else {
+        return b;
+    }
+}
+
+func Main() -> int {
+    let m1: int = Max<int>(10, 20);
+    let m2: int = Max<int>(5, 3);
+    
+    Std_Io_PrintLine("Max(10, 20) = ");
+    Std_Io_PrintInt(m1);
+    Std_Io_PrintLine("");
+    
+    Std_Io_PrintLine("Max(5, 3) = ");
+    Std_Io_PrintInt(m2);
+    Std_Io_PrintLine("");
+    
+    return 0;
+}