fix(selfhost): generic struct compilation, explicit generic args, method calls, field access

- parser: add parserIsTypeArgListAhead() for Func<Type>(args) and Type<T> { ... }
- hir_lower: method call desugaring with typeName tracking, self keyword,
  ekField typeName propagation, variable type tracking, strip '*' suffix
  from pointer type names in method calls, fix externFuncs buffer overflow
  (64 -> 256 elements) that corrupted structCount
- qbe_backend: QBE_FindStruct, QBE_FieldOffset, QBE_TypeSize helpers;
  hStructInit field-by-field codegen; hFieldPtr with offset + auto-load;
  hAssign/hStore special case for hFieldPtr (generate pointer, not value);
  fix null as *HirNode -> null as *void for extraData comparisons

demo_app/src/Main.bux

@@ -0,0 +1,29 @@
+// Structs - Basic struct usage
+import Std::Io::{PrintLine, PrintInt};
+
+
+struct Point {
+    x: int;
+    y: int;
+}
+
+func AddPoints(a: Point, b: Point) -> Point {
+    let result: Point = Point { x: a.x + b.x, y: a.y + b.y };
+    return result;
+}
+
+func Main() -> int {
+    let p1: Point = Point { x: 10, y: 20 };
+    let p2: Point = Point { x: 5, y: 15 };
+    let sum: Point = AddPoints(p1, p2);
+    
+    PrintLine("Point sum:");
+    PrintLine("x = ");
+    PrintInt(sum.x);
+    PrintLine("");
+    PrintLine("y = ");
+    PrintInt(sum.y);
+    PrintLine("");
+    
+    return 0;
+}