auto-exposure: spatial center-weighting, tunable mid-gray/max-exposure; psycho17 as default

[TheGreatHMMMM]

Summary

Replaces per-bin log-space Gaussian weighting in the resolve pass with spatial Gaussian center-weighting applied in the histogram pass. Adds two new user-tunable controls (targetAdaptedYf, maxExposure). Sets Psycho17 as the default tonemap operator.

Auto-exposure changes

Spatial center-weighting (auto_exposure_histogram.comp.slang)

• Each pixel contributes a Gaussian-weighted count to its histogram bin based on its distance from the screen centre (σ = 0.25 × short axis, fixed-point scale 256, minimum weight 1). Peripheral pixels still register, but the adaptation is biased toward the subject at the centre of gaze — matching real camera center-weighted metering.

Resolve pass (auto_exposure.comp.slang)

• Removed the per-bin log-space Gaussian (kGaussianSigmaStops, kGaussianLogCenter, kGaussianInvTwoSigmaSq, kTargetAdaptedYf). Bin counts from the histogram pass already carry the spatial weighting; the resolve now does a plain count-weighted log mean.
• targetAdaptedYf is now a push constant (cb.targetAdaptedYf) instead of a compile-time constant, allowing runtime adjustment.
• maxExposure clamp is applied to rawTargetExposure before the temporal blend, preventing integrator wind-up against the cap.
• Debug histogram normalization uses float(cb.numPixels) instead of the hard-coded 10,000.

New RTX options (rtx_auto_exposure.h, rtx_auto_exposure.cpp, tonemapping.h)

• rtx.autoExposure.targetAdaptedYf (default 0.18) — mid-gray adaptation target. Raise to bias the image brighter, lower to darken.
• rtx.autoExposure.maxExposure (default 8.0) — hard ceiling on the auto-exposure multiplier. Prevents dark rooms from being blown to mid-gray.
• Both are exposed as ImGui sliders and packed into ToneMappingAutoExposureArgs (replacing the three padding fields).

Tonemap operator changes

• Default operator changed from None to Psycho17.