Add AMD FSR2 SDK detection and backend integration scaffolding

docs/AMD_FSR2_INTEGRATION.md

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+# AMD FSR2 Integration Notes
+
+This project currently has two FSR2 states:
+
+- `AMD FidelityFX SDK` backend (preferred): enabled when SDK sources are present.
+- `Internal fallback` backend: used when SDK is missing.
+
+## SDK Location
+
+Drop AMD's official FSR2 repo at:
+
+`extern/FidelityFX-FSR2`
+
+Expected header for detection:
+
+`extern/FidelityFX-FSR2/src/ffx-fsr2-api/ffx_fsr2.h`
+
+## Build Flags
+
+- `WOWEE_ENABLE_AMD_FSR2=ON` (default): try to use AMD SDK if detected.
+- `WOWEE_HAS_AMD_FSR2` compile define:
+  - `1` when SDK header is found.
+  - `0` otherwise (fallback path).
+
+## Current Status
+
+- CMake detects the SDK and defines `WOWEE_HAS_AMD_FSR2`.
+- UI shows active FSR2 backend label in settings.
+- Runtime logs clearly indicate whether AMD SDK is available.
+
+## Remaining Work (to finish official AMD path)
+
+1. Add backend wrapper around `FfxFsr2Context` and Vulkan backend helpers.
+2. Feed required inputs each frame:
+   - Color, depth, motion vectors
+   - Jitter offsets
+   - Delta time and render/display resolution
+   - Exposure / reactive mask as needed
+3. Replace custom compute accumulation path with `ffxFsr2ContextDispatch`.
+4. Keep current fallback path behind a runtime switch for safety.
+5. Add a debug overlay:
+   - Backend in use
+   - Internal resolution
+   - Jitter values
+   - Motion vector validity stats
+6. Validate with fixed camera + movement sweeps:
+   - Static shimmer
+   - Moving blur/ghosting
+   - Fine geometry stability
+