design: convert Themes/Controls.xaml to tokens (surface 1 of the sweep)

The theme file that every control inherits is now token-driven: 73 hardcoded colours became
Brush.* references, and the two role changes the standard requires landed here first.

- Red leaves the chrome. The seven #e94560 sites (base button, focus borders, checked box and
  segment backgrounds, the selected-layer accent strip) were all interactive/selection roles,
  so they are blue now. Red stays for live/record only.
- The slider was green: its fill and gloss knob are blue now, with the track on
  SurfaceContainerLowest. The old green fill predated the standard.
- The pill toggle was ON = green, OFF = red — a settings control using the live colour. It is
  now OFF = neutral surface, ON = blue.
- Added scrim-soft/scrim-strong tokens (#22FFFFFF/#26FFFFFF) for the existing white washes, and
  Color.* resources for the two gradients (a GradientStop needs a Color, not a Brush).

Guard: DesignTokensTests gains Converted_surfaces_carry_no_raw_colour — a declared list of
converted files, so each sweep commit is guarded as it lands instead of waiting for one global
grep at the end. Pure white/black highlights are the only allowed literals.

No layout, spacing or behaviour change: this unit is colour only.

DESIGN.md

@@ -25,6 +25,8 @@
 | `surface-bright` | `#31394d` | Lightest surface, rare use (strong hover) |
 | `outline` | `#334155` | Borders, dividers |
 | `outline-variant` | `#1e293b` | Subtle dividers, inactive meter segments |
+| `scrim-soft` | `#22FFFFFF` | Hover/press wash over a surface (13% white) |
+| `scrim-strong` | `#26FFFFFF` | Stronger wash — selected icon, dimmed image (15% white) |
 
 ### Text