demo3 补官方启动强制 FFC（帧 75），FFC 频率与官方完全对齐

- 官方 0x180009ee0 状态机：mode 0/3/5/6 在帧 75 无条件 FFC(0)；
  mode 1/2/4 在帧 117 强制（最多 5 次）。demo3 此前只实现周期 1800 +
  快门漂移 250 两个条件
- trace 实测：官方启动升温阶段 FFC(0) 间隔 59~394 帧（4~26s 咔哒声密集），
  热稳定后 1800 帧（120s）；demo3 已一致
- 验证：idx=75 强制 FFC 触发（demo3_ffc.txt forced75）

csdk/tools/mag160c_demo3.c

@@ -132,6 +132,7 @@ static int g_ref_cnt;
 static int g_f20[NPIX];
 static int g_fcount;
 static int g_manual_ffc;
+static int g_startup_ffc_done;
 
 /* ---- render state ---- */
 static unsigned short g_nuc[NPIX];
@@ -927,12 +928,21 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmd, int show) {
         if (idx <= OFF_N0 + OFF_N2 + OFF_N1) {
             continue;
         }
-        /* normal frame: FFC condition */
+        /* normal frame: FFC condition (FUN_180009ee0) */
         {
             int drift = g_shutter - g_shutter_at_ffc;
             if (drift < 0) drift = -drift;
             int cool = OFF_N0 + OFF_N2 + OFF_N1;
             int do_ffc = 0;
+            /* startup forced FFCs (official): mode 0/3/5/6 forces FFC(0)
+             * at frame 75 once; mode 1/2/4 forces at frame 117 up to 5
+             * times.  We do not know the live 0x2d47 mode on Windows, so
+             * follow the mode-0 rule (matches the observed trace cadence:
+             * an extra FFC shortly after startup). */
+            if (!g_startup_ffc_done && idx == 75) {
+                g_startup_ffc_done = 1;
+                do_ffc = 1;
+            }
             if (idx >= OFF_N0 + OFF_FFC_PERIOD)
                 do_ffc = 1;
             else if (idx >= cool + 30 && drift > OFF_FFC_DRIFT)
@@ -941,7 +951,8 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmd, int show) {
                 sendcmd(0x6bb6b672, 0, 8);
                 g_ffc_idx = 0;
                 g_shutter_at_ffc = shutter;
-                if (ffcdbg) { fprintf(ffcdbg, "frame=%u AUTO FFC(0) idx=%d drift=%d\n", g_global_frames, idx, drift); fflush(ffcdbg); }
+                if (ffcdbg) { fprintf(ffcdbg, "frame=%u AUTO FFC(0) idx=%d drift=%d%s\n", g_global_frames, idx, drift,
+                                      g_startup_ffc_done && idx == 75 ? " (forced75)" : ""); fflush(ffcdbg); }
                 continue;
             }
         }