docs: record the per-scene render provenance difference

README.md

@@ -69,6 +69,23 @@ Clips are 11 s to 8.5 min of continuous motion — no cuts, no teleports.
 * **Panoramic frames are equirectangular.** A rectangular crop is *not* a perspective view —
   reproject before comparing against the first-person clip.
 
+## Render provenance
+
+`description.json` records the exact render path per clip, and it is **not identical across
+scenes**. Five scenes (med_village, venice, office, apartment, art_nouveau) were rendered in
+two rounds — `MPPC_RGBOnly` then `MPPC_DepthPlus` — and stitched with a Lanczos kernel, so
+`render.mppc` is an object with `rgb`/`depth` keys. **paris** was re-rendered later through a
+corrected single-pass path (`MPPC_VelocityDepthPlus` with the extra pass dropped, linear-HDR
+output, fixed exposure, linear stitch kernel), so its `render.mppc` is a plain string and its
+`render.cube` additionally carries `fixed_ev` and `tm_comp`.
+
+The reason for the paris re-render: a Lanczos resampling kernel has negative lobes, which on
+high-contrast sky/architecture boundaries interpolate linear HDR below zero; clamping to zero
+left a one-pixel pure-black rim along building silhouettes. Switching to a linear kernel took
+sky edges with undershoot from 15.5% to 3.4% — below the 6.1% baseline of the cube faces
+themselves. Per-frame auto-exposure was replaced with a fixed value at the same time, so
+brightness no longer drifts with what happens to be in view.
+
 ## Tools
 
 ```bash