Add screen-space ambient occlusion

[tomconder]

Summary

• Hemisphere-kernel SSAO (Crytek/Rendu-style), sampled against the depth prepass's depth and a new view-space normal MRT target, depth-aware blurred, then multiplied into the ambient term in pbr.slang.
• Depth prepass extended to also write view-space normals; light cubes (position-only geometry) read as "no surface" via a zero-normal sentinel.
• Debug UI: new SSAO tab (enable toggle, radius slider), and the old flat "Ambient Occlusion" scalar slider is removed — it was a placeholder for real AO that's now redundant, so SceneAmbient::occlusion defaults to 1.0 (no extra attenuation) and stays available as an optional multiplier on top of SSAO for artist tuning.

Fix folded in

While bringing up the feature, SSAO's raw/blur render targets read back as all-zero regardless of the occlusion math, which showed up as the whole scene going black with SSAO enabled. Root cause: the renderer leaves GL_BLEND enabled globally, the SSAO targets are single-channel GL_R16F (no alpha), and the shader's undefined alpha meant every write blended away to the framebuffer's zeroed initial contents instead of landing. Ssao::process() now disables blending for the pass, the same way the depth prepass already does for its own MRT writes.

Test plan

• Clean build (MSVC release + debug), no new compiler warnings.
• pre-commit run --all-files passes (clang-format, cpplint, shellcheck, etc.).
• Manual visual check at true default scene lighting (0 point lights): SSAO on shows full brightness with correct, subtle contact-shadowing in corners and under geometry; SSAO off matches pre-feature brightness.
• Toggled SSAO on/off and the radius slider live via the debug UI.