app+csdk+docs: 修正上一轮最近邻2×的退化（颗粒/边沿台阶）——照片改为从160×120单级6×放大（官方ImageViewer同款），实时缓冲与csdk回退为插值2×；实测冷区|dx| 0.76×官方（更平滑），119单测全绿

android/app/src/main/kotlin/com/mag160c/thermal/media/PhotoSaver.kt

@@ -50,12 +50,25 @@ import com.mag160c.thermal.core.AnnotSpec
 object PhotoSaver {
     private val TIME_FMT = SimpleDateFormat("yyyyMMdd_HHmmss", Locale.ENGLISH)
 
-    /** Sensor frame size used by the pipeline. */
-    const val SENSOR_W = 320
-    const val SENSOR_H = 240
+    /**
+     * Native sensor frame size (160x120), the SAME grid the probes/extremes and the
+     * MDT's NUC block use.
+     *
+     * It used to be 320x240 - the pipeline's internally 2x-upscaled buffer - and the
+     * photo was then scaled 3x on top of that: a two-stage upscale whose intermediate
+     * stage either interpolates (a full/half amplitude column pattern at the sensor's
+     * Nyquist) or replicates (halving the effective resolution: 2-sensor-pixel
+     * blocks, visible as a coarse staircase along hot/cold edges).  The official app
+     * upscales ONCE from the native 160x120 (`ImageViewer` draws the native bitmap
+     * into a 640x480 buffer with a filtering paint), so the saved photo is now a
+     * single 6x bilinear step from the sensor grid: 1-sensor-pixel granules, no
+     * interlace pattern.  [RENDER_SCALE] keeps the output at the same 960x720.
+     */
+    const val SENSOR_W = 160
+    const val SENSOR_H = 120
 
     /** Saved photo is this many times the sensor frame (crisp text). */
-    const val RENDER_SCALE = 3
+    const val RENDER_SCALE = 6
 
     fun fileName(now: Date = Date()): String = "MAG160C_${TIME_FMT.format(now)}.jpg"
 

android/app/src/main/kotlin/com/mag160c/thermal/ui/live/LiveViewModel.kt

@@ -658,6 +658,21 @@ class LiveViewModel(app: Application) : AndroidViewModel(app) {
      * stays in one fixed space rather than being resampled, and the rotation lives
      * only in the presentation layer.
      */
+    /**
+     * Exact 160x120 sensor-grid copy of the 320x240 display buffer. The pipeline's
+     * 2x upscale keeps the source value at every even index (`o[2x] = src[x]`), so
+     * sampling (2x, 2y) recovers the native pixels without any resampling.
+     */
+    private fun sensorFrame(display: IntArray): IntArray {
+        val out = IntArray(160 * 120)
+        var i = 0
+        for (y in 0 until 120) {
+            val row = y * 2 * 320
+            for (x in 0 until 160) out[i++] = display[row + x * 2]
+        }
+        return out
+    }
+
     fun capturePhoto(context: android.content.Context, density: Float = 2f) {
         val frame = latestFrame ?: return
         val s = session
@@ -690,8 +705,11 @@ class LiveViewModel(app: Application) : AndroidViewModel(app) {
             )
         }
 
+        // PhotoSaver works in the NATIVE sensor grid (160x120) and upscales once, so
+        // hand it the exact sensor pixels: the pipeline's 320x240 buffer is a 2x
+        // upscale whose even samples are the untouched source values.
         val jpg = com.mag160c.thermal.media.PhotoSaver.encodeRendered(
-            frame = frame,
+            frame = sensorFrame(frame),
             mirror = mirror,
             rotDeg = rotDeg,
             probes = st.probes.mapNotNull { p ->

android/app/src/main/kotlin/com/mag160c/thermal/usb/IrSession.kt

@@ -324,10 +324,13 @@ class IrSession(context: Context) {
             // anchor, no auto-centre search).  See RenderPipeline.lutRebuildAndroid
             // and docs/android_app/rendering_diff_phone_20261006.md §10.
             androidLut = true,
-            // the official Android display path has no interpolating 2x stage
-            // (nearest-neighbour draw of the native 160x120 bitmap); see the doc
-            // comment on RenderPipeline.upscaleNearest
-            upscaleNearest = true,
+            // Keep the interpolating 2x for the live buffer: the nearest variant
+            // halves the effective resolution (2-sensor-pixel granules, a visible
+            // staircase along hot/cold edges - measured, and reported by the user),
+            // while the interpolated one keeps 1-sensor-pixel granules. The photo no
+            // longer depends on this buffer: it is a single 6x step from the sensor
+            // grid (PhotoSaver.SENSOR_W/H = 160x120).
+            upscaleNearest = false,
             // baselineFrames stays at the verified 4: extending the window to 8 made
             // the output *unstable* (frame-to-frame gray noise 3.95 -> 8.8), i.e. the
             // device re-opens the shutter earlier than our schedule assumes, so the

android/app/src/test/kotlin/com/mag160c/thermal/media/PhotoNucMappingTest.kt

@@ -24,8 +24,13 @@ class PhotoNucMappingTest {
     fun renderScaleIsAppliedToTheSavedPhotoSize() {
         // keeps the sharpness fix pinned: text drawn at 1x was visibly soft
         assertTrue("photo is rendered larger than the sensor frame", PhotoSaver.RENDER_SCALE >= 2)
-        assertEquals(320, PhotoSaver.SENSOR_W)
-        assertEquals(240, PhotoSaver.SENSOR_H)
+        // The frame handed to the writer is the NATIVE sensor grid, not the
+        // pipeline's 2x display buffer: the photo is then a single upscale (no
+        // two-stage interlace, no halved effective resolution).
+        assertEquals(160, PhotoSaver.SENSOR_W)
+        assertEquals(120, PhotoSaver.SENSOR_H)
+        assertEquals("output stays 960x720", 960, PhotoSaver.SENSOR_W * PhotoSaver.RENDER_SCALE)
+        assertEquals(720, PhotoSaver.SENSOR_H * PhotoSaver.RENDER_SCALE)
         // the saved photo therefore stays 4:3 landscape — the sensor's own
         // orientation, with no display rotation baked in
         assertTrue(