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>

probes.yaml

@@ -15,6 +15,10 @@ defaults:
   # Baseline for a standalone FT2232H JTAG probe: drive ADBUS4 as the
   # "JTAG buffer enable" output (matches the built-in default).
   PROBE_FTDI_SET_PIN_DIR_ADBUS4: 1
+  # Driver-neutral JTAG clock in kHz. Honoured by the FTDI driver (mapped
+  # to PROBE_FTDI_TCK_FREQ_KHZ at open) and our Digilent driver. Leave it
+  # out to keep each driver's own default (FTDI 1000, Digilent 4000).
+  # JTAG_TCK_FREQ_KHZ: 1000
 
 profiles:
   # Embedded FlashPro on Microsemi eval kits (FT4232H, JTAG on channel A