fix(export): stop forcing a 0.5 full-res unsharp mask on every capture

App.tsx passed `sharpen: true` on camera exports, and both engines turned
that into a 0.5 unsharp mask whenever the SHARPENING knob sat at its
default 0. The 3x3 kernel [0,-a,0; -a,1+4a,-a; 0,-a,0] with a=0.5 has a
noise gain of 3.167x at low frequency and 5.0x at Nyquist, so high-ISO
files came back far noisier than the viewfinder ever showed.

The knob is now the only source of output sharpening: dropping the
fallback cuts flat-region noise sigma ~1.6x at ISO ~330 (2.36 vs 4.52)
and ~2.0x at ISO ~1900 (0.23 vs 0.46), and the JPEGs are 26-35% smaller.

Measured on a Xiaomi 12S Ultra, 3000x4000 captures, same scene:
  1x, ISO 335: sigma 2.35, dark-quartile 1.56, flat-tile 0.349
 10x, ISO 2119: sigma 1.09, dark-quartile 0.54, flat-tile 0.225

The SHARPENING knob still sharpens as before when dialled above 0.

src/utils/nativeExport.ts

@@ -24,9 +24,6 @@ export interface NativeExportResult {
 export interface NativeExportOptions {
   // DPI to stamp into the JPEG header (metadata only — pixel size never changes).
   dpi?: number;
-  // Apply the subtle "Screen" output sharpen (exportEngine #9) to the final
-  // pixels before encoding (legacy camera captures pass sharpen: true → 0.5).
-  sharpen?: boolean;
   // True for photos captured through the live camera: the AE bias already
   // applied exposure compensation at capture, so no 2^EV matrix gain is added.
   evFromCamera?: boolean;
@@ -73,11 +70,11 @@ export async function processAndExportPhotoNative(
     // (EXPO_PUBLIC_NATIVE_EXPORT is not inlined into a release HBC bundle).
     const tone = getToneUniforms(adjustments, recipe.baseFilter);
     const cinema = getCinemaUniforms(recipe.cinema);
-    // Final screen sharpen (engine #9): the sharpening knob (-10..10) overrides
-    // the legacy boolean sharpen flag (camera captures keep it true → 0.5).
+    // Final screen sharpen (engine #9): the sharpening knob (-10..10) is the
+    // only source — the old 0.5 fallback on every camera capture multiplied
+    // sensor noise ~3.2x and made dim/high-ISO shots unusable.
     const sharpKnob = adjustments.sharpening ?? 0;
-    const sharpenAmount =
-      sharpKnob > 0 ? (sharpKnob / 10) * 0.8 : sharpKnob < 0 ? 0 : options?.sharpen ? 0.5 : 0;
+    const sharpenAmount = sharpKnob > 0 ? (sharpKnob / 10) * 0.8 : 0;
     const aspect: '4:3' | '3:2' | undefined =
       options?.aspect && options.aspect !== 'full' && frameId !== 'wallframe'
         ? options.aspect