csdk: 撤回第29轮'关 pivot'的取值（依据已被真机反汇编推翻），C 参考保持 PC 语义；README 记录 Android 版 LUT 与 fpaTempFix 的待移植规格

csdk/tools/mag160c_linux_demo.c

@@ -125,11 +125,9 @@ int main(int argc, char **argv) {
     cfg.ffc_drift = 250;
     cfg.startup_force_ffc = 1;
     cfg.warm_frames = 20;
-    /* Official PHONE app runs the gray CDF WITHOUT the 75% pivot: with it on the
-       rendering is measurably darker than the official app (gray median 33 vs 61,
-       mean 60 vs 82 in a same-moment on-device comparison); off matches (50-54 vs 61,
-       80-81 vs 82).  See docs/android_app/rendering_diff_phone_20261006.md. */
-    cfg.cdf_pivot_75 = 0;
+    /* PC/Linux reference LUT keeps its own pivot (see the same note in
+       mag160c_demo3.c); the Android-parity LUT lives in the Android pipeline. */
+    cfg.cdf_pivot_75 = 1;
     cfg.ffc_cb = ffc_cb;
     mag160c_render_t *render = NULL;
     if (mag160c_render_init(&render, &cfg) != MAG160C_OK) {