Add LayoutContext, pointer types, non-struct pseudo-member, opaque type handling

Refactor block layout validation to use a LayoutContext struct that
threads module, definitions, pointer_size, and capabilities through
all layout calculation functions. This replaces individual parameter
passing and enables proper capability-based handling of special types.

- Add LayoutContext struct matching C++ ValidationState_t threading
- Add compute_pointer_size() from addressing model (Physical32 → 4,
  Physical64/PhysicalStorageBuffer64 → 8, Logical → 0)
- Add pointer type handling in type_alignment and type_layout_size
  (returns None for logical addressing, matching C++ behavior)
- Add non-struct pseudo-member path in check_struct_layout for
  PhysicalStorageBuffer pointers (C++ checkLayout lines 498-507)
- Add opaque type (image/sampler) handling with capability checks
  (BindlessTextureNV → needs samplerimage_variable_address_mode,
  DescriptorHeapEXT → not in rspirv yet, both skip gracefully)
- Add 6 new tests: pointer member under logical/physical addressing,
  pointer alignment validation, pointer overlap detection,
  non-struct type layout pass-through

Author: LegNeato