web: make EXPOSURE, EV and HIGHLIGHT mean what Lightroom means

A stop is a multiplier on light, so EXPOSURE and EV stop living in the sRGB
colour matrix and get a linear-light pass of their own (EXPOSURE_SKSL:
linearise, `C * 2^EV`, re-encode). The matrix keeps CONTRAST: a gain on encoded
values is what made +1 EV land at x1.5 instead of x2. Measured on the neutral
PROVIA sim: EV +1 = x2.011, EV +2 = x3.999, still unclipped at 239.

The pass sits between the matrix and the tone shader, and the tone / cinema /
curve / glow / halation children all sample through it, so HIGHLIGHT finally
sees the value exposure produced instead of the one before it. Recovery keeps
`L + strength * mask * (1 - L)` over `smoothstep(0.50,1.00,luma)`, and the
colour comes back as `color * (luma_new / luma)`: a blown white stays white
(255 -> 255 at -10, 255 at +10), a 0.8 grey loses 33 luma, the midtones beside
it do not move.

AUTO is the histogram the LIGHT tab already draws: weighted mean luminance
(guard 0.001), target 0.48, `log2(0.48 / avg)` clamped to +-2.5 EV, handed to
the same knob. A 0.251 grey asks for EV 0.9 and lands at mean 83.0 against the
83.3 predicted, idempotent on a second press. A stock's own bias rides the same
pass (`SIM_EXPOSURE_BIAS_EV`, VIVID +0.25 EV) and cancels against the knob, so
-1 EXPOSURE on VIVID returns the CLASSIC rendering (measured 0.4149 vs 0.4177).

ponytail: the phone app's `src/utils/colorUtils.ts` keeps the old math, so the
two copies have to move together; recipes saved before this commit (EXPOSURE 2,
HIGHLIGHT +-1) render under the new stop semantics.

Verified on the rebuilt container (BASE=http://localhost:8090):
- web-exposure-probe.cjs 20 PASS / 0 FAIL (neutral 128 -> 128, EV +1 ratio
  2.011, EV +2 ratio 3.999, EXPOSURE +10 ratio 5.62 / -10 ratio 0.172, AUTO
  EV 0.9, HIGHLIGHT -10 on a 204 grey 204 -> 171, white 255 -> 255, no console
  errors)
- sim-exposure-test.cjs 9 PASS / 0 FAIL (classic 0.4149, vivid 0.4531, knob -1
  returning 0.4177, bias 0.0382)
- regression suite, 28 probes: mask 53/0, brush-edit 35/0, heal-idle 23/0,
  heal-zoom-drag 28/0, sims 31/0, sim-vivid 9/0, white-black 4/0, temp-swatch
  33/0, tone-curve clean, compare 25/0, create 52/0, wb-preset 33/0, zoom 25/0,
  save-recent 25/0, web-smoke 9/0 (its export step was stale — EXPORT opens a
  size picker now). panel-test 4 FAIL, histogram-wb 1 FAIL, studio-save-hl and
  progate timeouts, landing-test 6 FAIL ($0.99 pricing) are pre-existing.
- npx tsc --noEmit clean.

docker/frontend/shared/utils/toneShader.ts

@@ -10,15 +10,20 @@ import { HSL_BANDS, hslBandGaps, isMonochromeBase } from './colorUtils';
 // 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 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.
+// at the very ends: HL rides the top (0.50..1.00) so it leaves the greys alone
+// (a knee that started lower dragged a mid-grey down) while SH rides the lower
+// half (0.00..0.55), and the 0.50 midpoint never moves.
 //
-// Both knobs are pure additive shifts of the luma. That keeps the curve
-// 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
+// HL is Lightroom's highlight RECOVERY and it is weighted by the headroom that is
+// left, (1 - t): the move falls to zero as t reaches pure white, so a blown sky,
+// a lamp or a specular can be pulled back without the white going grey. The pull
+// peaks around t = 0.79 at 0.13 of the ramp for a full -10 — the edge of a cloud,
+// which is the detail there is to get back (measured, and checked for
+// monotonicity over the whole -1..1 rectangle of the two knobs).
+//
+// Both knobs are additive shifts of the luma. That keeps the curve monotonic
+// (worst slope +0.003 at t = 0.99 with HL +10 and SH -10, and the two knees
+// barely 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.
 //
@@ -136,10 +141,15 @@ 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(0.65, 1.00, t);
+  float hlMask = smoothstep(0.50, 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 * 0.22 * hlMask + sh * 0.34 * shMask;
+  // The (1.0 - t) headroom weight is the whole point of the highlight curve: at
+  // t = 1.0 the
+  // weight is 0, so the pull-back cannot touch a pure white (a sun, a bulb, a
+  // specular) and cannot turn it grey. A LIFT gets the same weight, which rides
+  // it into the upper midtones and leaves the clipping where it was.
+  float o = t + hl * hlMask * (1.0 - t) + 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;
@@ -224,6 +234,37 @@ ${BAND_BLOCK}    hsl.x = fract(hsl.x + acc.x * (30.0 / 360.0));
 }
 `;
 
+// EXPOSURE / EV — the one pass that has to run in LINEAR light.
+//
+// A stop is a multiplier on LIGHT, and the old EV row multiplied sRGB-ENCODED
+// values: +1 EV took a mid-grey 0.5 straight to a blown 1.0 where a real stop
+// gives 0.73. Here the pixel is linearised, scaled by 2^ev, and re-encoded —
+// which is what a camera does when the shutter stays open twice as long: every
+// value keeps its ratio, the highlights roll instead of flattening, and the
+// HIGHLIGHT knob still has something to pull back afterwards.
+//
+// It sits between the graded image and the tone shader (see exportEngine step 3),
+// so `ev` carries the EXPOSURE knob, the stock's own bias and the EV knob added
+// up in stops — the caller hands in one number.
+//
+// The transfer pair is the accurate one (0.04045/12.92 + 2.4, and its inverse):
+// the same constants colorUtils.planckianLinear uses on the WB side.
+export const EXPOSURE_SKSL = `
+uniform shader src;
+uniform float ev;
+vec3 toLinear(vec3 c) {
+  return mix(c / 12.92, pow((c + 0.055) / 1.055, vec3(2.4)), step(vec3(0.04045), c));
+}
+vec3 toEncoded(vec3 c) {
+  return mix(c * 12.92, 1.055 * pow(c, vec3(1.0 / 2.4)) - 0.055, step(vec3(0.0031308), c));
+}
+vec4 main(vec2 xy) {
+  vec4 c = src.eval(xy);
+  vec3 rgb = clamp(c.rgb, 0.0, 1.0);
+  return vec4(clamp(toEncoded(toLinear(rgb) * exp2(ev)), 0.0, 1.0), c.a);
+}
+`;
+
 // Bright Pass Filter for the HDF EFFECT pass (HDF), SkSL over a child image
 // shader — the pattern TONE_SKSL above already proved on device.
 //