- Bump axslcc dependency to 3.99.1
- Remove runtime sampler slot compaction in D3D11 RenderContext;
  compaction is now done at compile time by axslcc 3.99.1
- Use samplerInfo.binding directly instead of local compactSlot counter
- Add D3D11_COMMONSHADER_SAMPLER_SLOT_COUNT bounds checking
- Update axslc-spec.h comments: binding is backend-visible slot,
  preset_index is stable logical id independent of remapping
- Update hlsl-spec.md to document that physical sampler registers
  are assigned by axslcc, not hardcoded