Draw the colour disc inside its tile rather than past it

The disc was sized against its parents in percentages, and every box
between it and the tile is sized by what is in it — so the chain never
resolved, the brightness slider grew, and the tile scrolled with half a
disc showing. Container units are the tile's own height whatever sits in
between, so the widget now fits exactly.

Two guards behind it, since both are things a picture can fail silently:
nothing on a panel of pictures may overflow its tile, and Touch has to
actually make a control bigger while leaving what it publishes alone.

NOTEPAD.md

@@ -100,13 +100,8 @@ Deferring because out of scope is fine, but don't mention deferring than.
   across all 16 columns. Another tile needs a rearrangement first — worth doing together
   with re-authoring `PAGES` as a single section.
 - CHORE/TEST: no Playwright coverage for the colour tile — `tests/widgets.spec.ts` covers the
-  other controls. Wants a spec that drags the hue ring and asserts one publish on release.
-- CHORE/UI: the colour wheel's arrow keys move a fixed 5°, with no PageUp/PageDown or
-  Home/End. Fine for picking by eye; a panel that wants an exact hue has no coarse/fine step.
-- CHORE/UI: the colour wheel reads the pointer's angle only, never its distance from the
-  centre (marked `ponytail:` in `ColorWidget.tsx`), so a colour takes two gestures — the ring,
-  then saturation. A saturation-by-radius disc would need a second axis on a control that can
-  announce one value.
+  other controls. Wants a spec that drags the disc and asserts one publish on release. The
+  maths is checked (`color.check.ts`); what is not is the gesture.
 - CHORE/DEMO: `seed_demo.py` still writes the Home dashboard as three titled sections
   ("Right now", "Energy and comfort", "Yield model"). The editor and the panel now read a
   page's sections as one grid, so those headings are no longer drawn and the first editor