selfhost: auto-Drop for Array<T> in @[Checked] functions

In @[Checked] functions, variables of type Array<T> automatically get
a defer Array_Free_T(&var) injected at declaration site. This ensures
Array values are freed when they go out of scope, preventing leaks.

Implementation:
- HIR lowering detects Array_<T> types in skLet statements
- Generates Array_Free_T generic instance if needed
- Builds synthetic defer node: defer Array_Free_T(&var)
- Wraps let+defer in hBlock for proper statement chaining

Also adds checkedFunc flag to HirFunc and CEmitter for future use.

src/hir.bux

@@ -114,6 +114,7 @@ struct HirFunc {
     captureType7: int;
     envStructName: String;
     envInstanceName: String;
+    checkedFunc: bool;
 }
 
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