v0.43.0: visualizer presets

Each visualizer style's settings page has a Preset drop-down and a
"Save as Preset…" button. The drop-down lists the built-in Default, your
presets, and "Edit Presets…" (rename in place, delete); it reads "Custom"
whenever the settings match no preset. Presets sync with the preferences;
the per-machine sync delay is never part of one.

CLAUDE.md

@@ -224,6 +224,14 @@ persistence) → GUI (Qt widgets that read the manager and connect to its signal
   lives in **`config.json`**; the dialog emits `prefs.changed` after writing
   it so the widget re-reads. The laser takes every heard sample exactly once
   (`_drawn` index), never the newest-decoded ones.
+  **Presets** (`gui/visualizer_presets.py`): per style, `STYLE_KEYS` under a
+  name, in `visualizer_presets` (synced; never the sync delay). `Default` is
+  built in and reserved. Applying one moves the dialog's *controls* (each
+  registers a setter in `_viz_setters`), and their own handlers save the
+  values, so controls and prefs can't disagree. The drop-down reads
+  "Custom" whenever the settings match no preset. A setting added later
+  must join `STYLE_KEYS` and register a setter, or presets silently skip it
+  (`test_every_style_key_has_a_control` pins that).
   The highlight swatches can be repainted with a color picker:
   `highlight_palette` overrides a slot, and `theme.highlight_color` reads
   through `theme.highlight_palette`, so nothing else needs to know.