Record that exports of oversized linear DNGs now tile

FR-DSP-2 and NFR-RES-2 said tiling was unbuilt; the export half of it
now exists for a linear DNG past one texture. The interactive path and
spike S6 are unchanged.

docs/dev/outstanding.md

@@ -200,6 +200,10 @@ Two measurements say it costs more than it saves on the interactive path. Neithe
 about the export path or about a device under memory pressure, which is where the case for it
 actually lives — and that is spike S6, which has not run.
 
+**2026-09-27:** the export path does now tile, for the one case that forced it — a linear DNG
+wider than any texture. See the note under FR-DSP-2 in [requirements.md](requirements.md). The
+interactive path is unchanged and S6 is still unrun.
+
 **FR-DSP-4 — Progressive refinement. Built in 0.15.0.** While a gesture moves the canvas renders
 a half-resolution draft, and the sharp frame lands once, 120 ms after the last movement: the
 decision is `ui/dr-ui/src/refine.rs`, a debounce whose every draft re-arms the settle timer, driven