jtag: driver-neutral JTAG_TCK_FREQ_KHZ clock (phase A)

One clock knob across probes instead of per-driver names:
- jtag_open mirrors JTAG_TCK_FREQ_KHZ into PROBE_FTDI_TCK_FREQ_KHZ for
  the Viveris FTDI driver (read-only at init); unset leaves the existing
  value untouched
- the Digilent driver reads JTAG_TCK_FREQ_KHZ directly instead of
  hardcoding 4 MHz (falls back to 4 MHz when unset)
- documented in probes.yaml; CLAUDE.md design note marks phase A done

FTDI path validated on the IGLOO2/FlashPro (250 kHz, mirror confirmed);
Digilent path not hardware-tested.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

modules/drivers/digilent_jtag/digilent_jtag_drv.c

@@ -15,6 +15,7 @@
 #include "jtag_core/jtag_core.h"
 #include "jtag_core/dbg_logs.h"
 #include "config/bs_defines.h"
+#include "script/env.h"
 
 #include "drivers/drv_loader.h"
 
@@ -209,11 +210,17 @@ static int drv_Digilent_Detect(jtag_core *jc)
 
 static int drv_Digilent_Init(jtag_core *jc, int sub_drv, char *params)
 {
-    DJ_DWORD frq_req = 4000000; /* 4 MHz — safe and fast enough for IDCODE/IR */
+    DJ_DWORD frq_req;
     DJ_DWORD frq_set = 0;
+    int      tck_khz;
 
     (void)params;
 
+    /* Driver-neutral JTAG clock (JTAG_TCK_FREQ_KHZ, kHz). Unset/<=0 keeps
+     * the 4 MHz default — safe and fast enough for IDCODE/IR. */
+    tck_khz = jtagcore_getEnvVarValue(jc, "JTAG_TCK_FREQ_KHZ");
+    frq_req = (tck_khz > 0) ? (DJ_DWORD)tck_khz * 1000u : 4000000u;
+
     /* Lazy enumeration: jtag_open can be called without going
      * through jtag_probes first, so make sure Detect ran. */
     if (g_dj_num_probes == 0) {