Split effects into image and video effects, with VHS tracking as the first video effect

Effect::video() says which kind an effect is. The Add effect menu lists the
two kinds in sections of their own, each scrolling on its own, and offers
video effects only in a video project; a video stage carries a film icon in
the chain and, in a still, is dimmed and skipped like a stage switched off.
enabledStages takes whether the render is a video and is the one place that
leaves video stages out of a still. --help groups the video effects under a
heading of their own, --list-effects marks them, and a video effect given
for a still is skipped with a warning.

VideoEffect is the base every video effect builds on, and keeps the rules a
moving stage has to follow: it takes one value from the shared rng, the same
on every frame, and hands the effect steady and per-frame generators and
loop-rounded time through a Moment, so a subclass writes only animate().
VHS tracking is the first, and the one to copy: the colour planes sway and
shake and a tracking bar rolls down the picture.

That motion moves out of channel split, which is an image effect again and
renders stills exactly as before; forVideo() goes with it. Version 0.7.0.

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

CHANGELOG.md

@@ -4,6 +4,26 @@ Every release of Effector, newest first. The version lives in one place, the
 `project()` line of `CMakeLists.txt`, and each release is a `vX.Y.Z` tag on the
 commit that made it. The interface shows this file under Changelog in its menu.
 
+## [0.7.0] - 2026-09-26
+
+### Added
+- Effects are image effects or video effects. The Add effect menu lists them
+  in two sections, each under its own heading, and offers video effects only
+  in a video project. In the chain a video stage has a film icon, and in a
+  still it is dimmed and skipped, like a stage that is switched off.
+- VHS tracking, the first video effect: the colour planes sway and shake, and
+  a tracking bar rolls down the picture. On the command line it is
+  `-e tracking` with `--tracking-wobble`, `--tracking-jitter`, `--tracking-bar`
+  and the rest.
+- `--help` lists the video effects under a heading of their own, and
+  `--list-effects` marks them. A video effect given for a still is skipped
+  with a warning.
+
+### Changed
+- Channel split is an image effect again. Its motion (Wobble, Jitter, Tracking
+  bar) has moved to VHS tracking, so a 0.6 video project that moved its channel
+  split needs a VHS tracking stage after it. Stills are unchanged.
+
 ## [0.6.1] - 2026-09-26
 
 ### Fixed