feat(V4B): V4.36.18 — 频率窗下界 20→18kHz

COIL_F_OK_MIN_KHZ 18: 合理窗 18~125kHz (@43nF ≈ L47µH~1.15mH) 放行更大电感
test 边界 17偏大/18正常 全绿; 镜像同步
顺带修正 spec 参数表历史滞后: MAX 115→125, COIL_CAL_C_NF 68→43
语法0 error; spec V2.34 / manual / devlog V2.53

tests/test_coil_range.c

@@ -3,7 +3,7 @@
  *
  * 验证对象: TaskLoop.c coil_monitor() (V4.26 拍板规格)
  *   默认低频档口径(43nF 三极管板): 空场频率合理窗 20~125kHz, 出界即判定电感量不合理
- *   g_coil_fault: 0=正常 1=L偏大(f<20kHz) 2=L偏小(f>125kHz)
+ *   g_coil_fault: 0=正常 1=L偏大(f<18kHz) 2=L偏小(f>125kHz)
  *   防抖: 越界/恢复均连续 100tick(=1s) 才置位/清除 (调用点=稳定无车空闲)
  *
  * 镜像宏与固件一致 (改 COIL_* 时同步此处!)
@@ -13,7 +13,7 @@
 #include <stdio.h>
 #include <stdint.h>
 
-#define COIL_F_OK_MIN_KHZ  20
+#define COIL_F_OK_MIN_KHZ  18
 #define COIL_F_OK_MAX_KHZ 125
 #define COIL_DEF_C_NF     66
 
@@ -85,9 +85,9 @@ static void test_recover(void)
 static void test_boundaries(void)
 {
     g_coil_fault = 0; g_coil_bad_cnt = 0;
-    run_ticks(19, 100);   assert(g_coil_fault == 1);   /* 19 → 偏大 */
+    run_ticks(17, 100);   assert(g_coil_fault == 1);   /* 19 → 偏大 */
     run_ticks(62, 100);   assert(g_coil_fault == 0);   /* 20~125 正常 */
-    run_ticks(20, 100);   assert(g_coil_fault == 0);   /* 边界 20 正常 */
+    run_ticks(18, 100);   assert(g_coil_fault == 0);   /* 边界 20 正常 */
     run_ticks(125, 100);  assert(g_coil_fault == 0);   /* 边界 125 正常 */
     run_ticks(126, 100);  assert(g_coil_fault == 2);   /* 126 → 偏小 */
     printf("4. 边界 19/20/125/126 OK\n");
@@ -111,6 +111,6 @@ int main(void)
     test_recover();
     test_boundaries();
     test_jitter_no_alarm();
-    printf("\nALL PASS (窗口 20~125kHz, 防抖 1s)\n");
+    printf("\nALL PASS (窗口 18~125kHz, 防抖 1s)\n");
     return 0;
 }