feat: generic enums, fix is-operator, Type_Eq, hardcoded limit diagnostics

- feat: generic enum support (parser, lowering, codegen — both selfhost + bootstrap)
  - enum Result<T,E> { Ok(T), Err(E) } parsing + monomorphization
  - tag constant mangling in monomorphized function bodies
  - data field access (l-value + r-value) for generated enum instances
  - multiple concrete instances in same file
  - HIR walker for enum reference mangling (selfhost + bootstrap)

- feat: stdlib Result<T,E> and Option<T> made truly generic
  - breaking: explicit type args required (Result<int, String>)

- fix: 'is' operator — lowering to hBinary tag comparison + C backend fallback
- fix: Type_Eq structural comparison (inner types for pointer/slice/tuple)
- fix: hardcoded limit diagnostics (>8 params/variants/captures now emit errors)
- docs: Iter<T> safety warning for dangling pointer
- docs: IMPROVEMENTS.md — comprehensive plan and changelog
- test: generic_enum example added to EXAMPLES

All tests pass (0 FAIL). Selfhost loop deterministic.

src/types.bux

@@ -162,9 +162,22 @@ module Types {
 
     func Type_Eq(a: Type, b: Type) -> bool {
         if a.kind != b.kind { return false; }
-        if a.kind == tyNamed || a.kind == tyTypeParam {
+        if a.kind == tyNamed || a.kind == tyTypeParam || a.kind == tyFunc {
             return String_Eq(a.name, b.name);
         }
+        if a.kind == tyPointer {
+            if a.innerKind1 != b.innerKind1 { return false; }
+            return String_Eq(a.innerName1, b.innerName1);
+        }
+        if a.kind == tySlice || a.kind == tyTuple {
+            if a.innerKind1 != b.innerKind1 { return false; }
+            if !String_Eq(a.innerName1, b.innerName1) { return false; }
+            if a.innerKind2 != b.innerKind2 { return false; }
+            if !String_Eq(a.innerName2, b.innerName2) { return false; }
+            if a.innerKind3 != b.innerKind3 { return false; }
+            if !String_Eq(a.innerName3, b.innerName3) { return false; }
+            return true;
+        }
         return true;
     }