Doc: Record 4K fast-mode accuracy limits and error distribution

Add the production-density 4K Galactic-center comparison to the 2026-09-25 benchmark record, including the per-image error decomposition, and refine the fast-mode positioning in usage.md, the design document, and README.

- Document that nearest/bilinear deposit accuracy is set by the deposit mode and N rather than output resolution, and that the still-frame error is dominated by a small number of resolved bright cores while the flux-dominant faint texture is much better than the global relative L2.
- Add the temporal-coherence caveat: nearest deposition jumps by 1/N output pixel per axis across supersampled-cell boundaries, so fast mode is not qualified as a temporally coherent movie path; bilinear removes the centroid jump but broadens the profile.
- Add scripts/fast_mode_error_decomposition.py, which ranks the largest |fast-base| RGB samples and partitions the error by a disjoint base-value range.

README.md

@@ -132,10 +132,12 @@ PNGs. The command above omits the optional HDR and lens-map exports.
 This example infers a position facing the hole from its look direction and radius. Adaptive refinement should
 be configured for the desired image accuracy; it is disabled by default.
 Camera controls, movie sequences, lens-map reuse, PSF settings, and HDR output
-are described in [usage.md](usage.md). For fast previews, `--fast-mode` replaces
-per-event PSF splats with a supersampled delta deposit plus one global PSF
-convolution and downsample. Both binaries provide a complete option list with
-`--help`.
+are described in [usage.md](usage.md). For fast single-frame previews,
+`--fast-mode` replaces per-event PSF splats with a supersampled delta deposit
+plus one global PSF convolution and downsample. Its nearest-deposit path is not
+qualified for temporally coherent movie output; see [usage.md](usage.md) for
+the spatial and temporal accuracy limits. Both binaries provide a complete
+option list with `--help`.
 
 ### Example: Looking outward just above the Schwarzschild horizon