Harden Large allocation layout invariants

[Copilot]

The internal Large allocation logic relies on precise layout and pointer offset invariants for soundness. This update tightens layout construction and validates header/data offsets across alignments.

• Layout construction
  • Use Layout::extend/pad_to_align to compute heap layouts directly from header + element layout.
• Pointer provenance
  • Derive data_start from the header size and document the invariant.
• Coverage
  • Add layout/data-start assertions across multiple element alignments with guarded deallocation.

Example:

let header = Layout::new::<Allocated<T>>();
let data = Layout::array::<T>(cap).expect("new capacity is too large");
let (layout, offset) = header.extend(data).expect("new capacity is too large");
debug_assert_eq!(offset, size_of::<Allocated<T>>());
layout.pad_to_align()

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