D3D12: Fix texture ownership semantics and remove GetSwapChain() exposure

This commit fixes the D3D12 texture architecture issues:

1. D3D12Texture ownership semantics:
   - Add m_ownsResource member to track resource ownership
   - InitializeFromExisting() now takes ownsResource parameter (default false)
   - Shutdown() only releases resource if ownsResource is true
   - Initialize() sets m_ownsResource = true for created resources

2. D3D12SwapChain changes:
   - Remove GetSwapChain() method (was exposing native D3D12 API)
   - Change GetBackBuffer() to return reference instead of pointer
   - Back buffers initialized with ownsResource = false

3. minimal/main.cpp updates:
   - Remove gColorRTs[2] array (was duplicating back buffer wrapping)
   - Direct use of gSwapChain.GetBackBuffer(i) instead
   - All references updated to use encapsulated API

4. Documentation:
   - Update TEST_SPEC.md v1.3
   - Remove known limitation 7.2 (minimal GetBuffer issue fixed)
   - Add D3D12_Texture_Architecture_Fix_Plan.md design document

engine/src/RHI/D3D12/D3D12Texture.cpp

@@ -33,11 +33,13 @@ bool D3D12Texture::Initialize(ID3D12Device* device, const D3D12_RESOURCE_DESC& d
         return false;
     }
 
+    m_ownsResource = true;
     return true;
 }
 
-bool D3D12Texture::InitializeFromExisting(ID3D12Resource* resource) {
+bool D3D12Texture::InitializeFromExisting(ID3D12Resource* resource, bool ownsResource) {
     m_resource = resource;
+    m_ownsResource = ownsResource;
     return true;
 }
 
@@ -178,6 +180,9 @@ bool D3D12Texture::InitializeDepthStencil(ID3D12Device* device, uint32_t width,
 }
 
 void D3D12Texture::Shutdown() {
+    if (m_ownsResource) {
+        m_resource.Reset();
+    }
     m_resource.Reset();
 }