Phase 8.2.4: Slice fat pointer + bounds checking

- T[] slices are now fat pointers {data: T*, len: size_t} in C backend
- Added hSliceIndex HIR node for slice indexing (distinct from hIndexPtr for raw pointers)
- Added exprIndexBoundsCheck flag to AST ekIndex
- Sema sets bounds-check flag in @[Checked] functions
- C backend emits Slice_<T> typedefs and compound literals for slice init
- bux_bounds_check() called automatically on slice indexing in @[Checked] funcs
- Unchecked functions allow raw access (C behavior) for performance

src/ast.nim

@@ -179,6 +179,7 @@ type
     of ekIndex:
       exprIndexObj*: Expr
       exprIndexIdx*: Expr
+      exprIndexBoundsCheck*: bool
     of ekField:
       exprFieldObj*: Expr
       exprFieldName*: string