Phase 16.6-16.7: Fix duplicate include and standardize include ordering

[Cle2ment]

Duplicate include:

• Renderer.cpp:8 and :11 both #include "CPUBackend.h" (line 8 is only needed under PN_ISPC, line 11 is unconditional)

Include ordering (3 files): project headers → framework headers → third-party → standard:

• Camera.cpp — Peanut input header after glm
• CPUBackend.cpp — Constants.h after standard headers
• OptiXDenoiser.cpp — cuda.h before cstdio

[github-actions]

All four issues are straightforward. Here's my guidance:

1. Renderer.cpp:8 — duplicate #include "CPUBackend.h"
Remove line 8 (the #ifdef PN_ISPC guarded one). Line 11 already covers the include unconditionally, and CPUBackend.h itself does not require PN_ISPC to be defined (its ISPCRenderPixels call is already guarded internally).

2. Camera.cpp:7 — "Peanut/Input/Input.h" after glm
Per AGENTS.md convention: project → framework → third-party → standard. Move line 7 (#include "Peanut/Input/Input.h") up to right after #include "Camera.h" (line 1), before the three glm includes (lines 3–5).

3. CPUBackend.cpp:11 — "Constants.h" after standard headers
Move the #include "Constants.h" on line 11 up, grouping it with the other project headers (PathTracerCore.h) right after line 4. Standard/library headers (<cstring>, <execution>, etc.) should remain together at the bottom of the include block.

4. OptiXDenoiser.cpp:5,11 — <cuda.h> before <cstdio>, with code between them
The ordering itself is fine (third-party <cuda.h> before standard <cstdio> per convention), but the #include <cstdio> on line 11 is separated from <cuda.h> by the global variable definition on line 9. Move #include <cstdio> immediately after #include <cuda.h> (line 6) so all includes are grouped together before any declarations.

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