Keep the highlight slider off the greys, and start each mode clean

The highlight knee opened at 0.45, so dropping HIGHLIGHT pulled a mid-grey
down with the true highlights. It now opens at 0.65 (and the coefficient
drops 0.32 -> 0.22 to stay monotonic), so the slider leaves everything up to
a light grey untouched and still rolls the bright end off. Shadow is
untouched — its 0.00..0.55 knee was already right.

Switching between the library and the camera now drops the edits back to
the recipe's own values, no frame and no custom mark. A frame was picked
for the still it sat on and the mark was placed against that very photo, so
neither may follow the user across the switch (or into a live capture),
exactly like the sliders that were tuned on the photo in front of them.

src/utils/toneShader.ts

@@ -9,14 +9,15 @@ import { ColorAdjustments } from '../types';
 // turned saturated blues black under -SH and bright colours grey under -HL.
 //
 // Wide, soft knees so the knobs reach like a tone curve instead of biting only
-// at the very ends: HL rides the upper half (0.45..1.00), SH the lower half
-// (0.00..0.55), and the 0.50 midpoint moves less than 3%. Earlier narrow
+// at the very ends: HL rides only the top (0.65..1.00) so it leaves the greys
+// and midtones alone (a 0.45 knee dragged a mid-grey down) while SH rides the
+// lower half (0.00..0.55), and the 0.50 midpoint never moves. Earlier narrow
 // knees (0.80..1.00 / 0.00..0.30) made both sliders look dead on any photo
 // without true whites or blacks.
 //
 // Both knobs are pure additive shifts of the luma. That keeps the curve
-// monotonic (steepest knee slope 2.73/unit * 0.32 = 0.87, +SH overlap still
-// positive), so a brighter input can never come out darker. The earlier
+// monotonic (steepest knee slope 4.29/unit * 0.22 = 0.94, and the two knees
+// never overlap), so a brighter input can never come out darker. The earlier
 // multiplicative form was NOT monotonic: with hl=-1 a grey 0.73 came out
 // darker than 0.80.
 //
@@ -38,10 +39,10 @@ vec4 main(vec2 xy) {
   vec4 c = src.eval(xy);
   vec3 rgb = clamp(c.rgb, 0.0, 1.0);
   float t = clamp(dot(rgb, vec3(0.2126, 0.7152, 0.0722)), 0.0, 1.0);
-  float hlMask = smoothstep(0.45, 1.00, t);
+  float hlMask = smoothstep(0.65, 1.00, t);
   float shMask = 1.0 - smoothstep(0.00, 0.55, t);
   // NOTE: never name a local 'out' — it is a reserved SkSL qualifier.
-  float o = t + hl * 0.32 * hlMask + sh * 0.34 * shMask;
+  float o = t + hl * 0.22 * hlMask + sh * 0.34 * shMask;
   // Dynamic range: gentle shadow lift + highlight roll (protect brights).
   o += dr * 0.12 * shMask * (1.0 - t);
   o -= dr * 0.18 * hlMask * t;