Add DPI scaling validation documentation

Co-authored-by: philstopford <1983851+philstopford@users.noreply.github.com>

Author: Copilot

DPI_SCALING_VALIDATION.md

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+# DPI Scaling Logic Validation
+
+## Scenario 1: Normal DPI (1x)
+- Window logical size: 800x600
+- RenderWidth: 800, RenderHeight: 600 (same as logical)
+- LogicalPixelSize: 1.0
+
+### Before Fix (Incorrect):
+- Orthographic projection used RenderWidth/RenderHeight (800x600)
+- Mouse coords scaled by LogicalPixelSize (1.0) - no effect
+- Result: Correct behavior by accident
+
+### After Fix (Correct):
+- Orthographic projection uses Width/Height (800x600)
+- Mouse coords NOT scaled
+- Result: Correct behavior by design
+
+## Scenario 2: High DPI (2x)
+- Window logical size: 800x600
+- RenderWidth: 1600, RenderHeight: 1200 (2x scaled)
+- LogicalPixelSize: 2.0
+
+### Before Fix (Incorrect):
+- Orthographic projection used RenderWidth/RenderHeight (1600x1200)
+  - View frustum: 1600 * zoom in world units wide
+  - Objects appear 2x smaller than intended
+- Mouse coords at (400, 300) logical pixels scaled to (800, 600)
+  - ScreenToWorld used RenderWidth/RenderHeight (1600x1200)
+  - Incorrect world coordinate calculated
+- ZoomExtents divides world extent by 1600, making objects appear smaller
+- Result: Geometry too small, not centered
+
+### After Fix (Correct):
+- Orthographic projection uses Width/Height (800x600)
+  - View frustum: 800 * zoom in world units wide
+  - Objects appear correct size
+- Mouse coords at (400, 300) logical pixels NOT scaled
+  - ScreenToWorld uses Width/Height (800x600)
+  - Correct world coordinate calculated
+- ZoomExtents divides world extent by 800, correct zoom level
+- Result: Geometry properly sized and centered
+
+## Mathematical Example
+
+World space geometry bounds: 0 to 1000 units in X
+
+### Normal DPI (1x):
+- Before: zoomLevel = 1000 / 800 = 1.25
+- After: zoomLevel = 1000 / 800 = 1.25
+- Same result ✓
+
+### High DPI (2x):
+- Before: zoomLevel = 1000 / 1600 = 0.625 (too zoomed out!)
+- After: zoomLevel = 1000 / 800 = 1.25 (correct!)
+- Fixed ✓
+
+## Key Insight
+
+The framebuffer size (RenderWidth/RenderHeight) should only be used when:
+1. Creating the swapchain (already done correctly by platform handlers)
+2. Allocating GPU resources
+
+For all viewport calculations (projection, zoom, coordinate conversion), we should use the logical dimensions (Width/Height) to maintain consistent behavior across different DPI settings.