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) {

modules/script/script.c

@@ -1863,6 +1863,20 @@ static int cmd_open_probe(script_ctx *ctx, char *line)
             ctx->script_printf(ctx, MSG_INFO_0, "Applied probe profile '%s'.\n", profile);
         }
 
+        // Driver-neutral JTAG clock: if JTAG_TCK_FREQ_KHZ is set, feed it to
+        // the Viveris FTDI driver's own variable (it reads only PROBE_FTDI_*;
+        // our Digilent driver reads the neutral name directly). Harmless for
+        // the other drivers, which keep their defaults.
+        {
+            int tck_khz = jtagcore_getEnvVarValue(jc, "JTAG_TCK_FREQ_KHZ");
+            if (tck_khz > 0)
+            {
+                char tck_str[24];
+                snprintf(tck_str, sizeof(tck_str), "%d", tck_khz);
+                setEnvVarDat((envvar_entry *)ctx->env, "PROBE_FTDI_TCK_FREQ_KHZ", tck_str);
+            }
+        }
+
         ret = jtagcore_select_and_open_probe(jc, id);
         if (ret != JTAG_CORE_NO_ERROR)
         {