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/hir.nim

@@ -24,6 +24,7 @@ type
     hFieldPtr
     hArrowField
     hIndexPtr
+    hSliceIndex
     # Functions
     hCall
     hCallIndirect
@@ -136,6 +137,11 @@ type
       structInitFields*: seq[tuple[name: string, value: HirNode]]
     of hSliceInit:
       sliceInitElements*: seq[HirNode]
+      sliceInitLen*: int
+    of hSliceIndex:
+      sliceIndexBase*: HirNode
+      sliceIndexIndex*: HirNode
+      sliceIndexBoundsCheck*: bool
     of hRange:
       rangeLo*: HirNode
       rangeHi*: HirNode