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

src/ast.nim

@@ -104,6 +104,7 @@ type
     ekTernary
     ekRange
     ekCall
+    ekGenericCall
     ekIndex
     ekField
     ekStructInit
@@ -160,6 +161,9 @@ type
     of ekCall:
       exprCallCallee*: Expr
       exprCallArgs*: seq[Expr]
+    of ekGenericCall:
+      exprGenericCallee*: string
+      exprGenericTypeArgs*: seq[TypeExpr]
     of ekIndex:
       exprIndexObj*: Expr
       exprIndexIdx*: Expr