HDRP sample looks washed out: over-exposed fixed EV + ACES tonemapping (not a plugin colour bug) #18

Symptom

samples/hdrp-singlepass-ui (Linear, HDRP, Unity 6000.4.0f1) looks washed out on the 3D panel: flat, low-contrast, desaturated crate under the HDRP sky.

It is NOT a plugin/runtime colour-format problem (A/B evidence, on hardware 2026-10-02)

Same player build, two plugin DLLs: before vs after the plugin colour fix DisplayXR/displayxr-unity#347 (PR DisplayXR/displayxr-unity#348).

• 3D scene unchanged: crate pixel (135,101,85) pre-fix vs (137,103,85) with #348 — atlas-capture pixels match within noise.
• HUD (wsui overlay) got darker with #348 — i.e. #348 corrected the overlay path, as intended. The scene path was already correct.

So the washed-out look of the 3D scene is the sample's own HDRP tuning, not the pipeline. (Plugin's #191 noted a "washed-out" HDRP look as "an HDRP lighting/exposure issue" as well — this is that, pinned down.)

Cause: the sample's HDRP tuning

1. Exposure — the scene's global volume (Assets/CubeTest/Sky and Fog Settings.asset) overrides exposure to Fixed, 12.696 EV100, replacing the default profile's Automatic exposure. For a 100,000 lux sun at ~50° elevation that is ~1.6 stops over (worked below / in the PR): an 18% grey lit face lands at ~0.54 scene-linear instead of ~0.18.
2. Tonemapping — Assets/HDRPDefaultResources/DefaultSettingsVolumeProfile.asset has Tonemapping = ACES, which compresses and desaturates the over-exposed mids/highlights → flat and washed out.
3. Colour buffer is R11G11B10 (colorBufferFormat: 74 in HDRenderPipelineAsset.asset) — fine, not part of the problem, leave it.

Fix plan

• Tonemapping ACES → Neutral.
• Exposure: keep Fixed (deterministic, so atlas-capture A/Bs are reproducible) but derive the value from the scene's actual light (sun 100,000 lux, Lux unit) instead of 12.7.
• Change nothing else (no lighting rebalance, no colour-space change).

Verification: Windows player build; panel check by eye + atlas capture.