Add Warp flow, a video effect that sets Warp's noise field drifting

Warp's noise field moves to effects/noise_field.hpp so both share it; Warp
renders exactly as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -17,6 +17,10 @@ commit that made it. The interface shows this file under Changelog in its menu.
   new echo fading in at the far end as the nearest melts into the picture.
   On the command line it is `-e tunnel` with `--tunnel-speed`,
   `--tunnel-scale`, `--tunnel-direction` and the rest.
+- Warp flow, a video effect: the picture pushed or swirled through a field
+  of noise as Warp does, with the field drifting so the picture ripples like
+  heat haze or water. On the command line it is `-e flow` with
+  `--flow-strength`, `--flow-travel`, `--flow-speed` and the rest.
 - A Quality choice under Video on Setup: Small file, Standard or High, for
   how the saved video is encoded. It is kept in the project, and on the
   command line it is `--video-quality small|standard|high`. Standard writes