jtag/fpga: prog method tag + RTCK link setting (phase C)

- fpga_target gains a prog method (proxy_spi/svf/none), set in the
  registry or inferred when omitted (proxy_bitstream -> proxy_spi;
  Microsemi/Lattice -> svf); shown by fpga_info/fpga_list and exposed via
  fpga_prog_method_name() for the future program dispatch
- generalise RTCK as a neutral JTAG_RTCK, mirrored to
  PROBE_FTDI_JTAG_ENABLE_RTCK at open (FTDI-only)
- reset abstraction deferred (no clean neutral form yet); the program
  dispatch command itself lands with the SVF player

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

modules/fpga/fpga.h

@@ -30,6 +30,13 @@ typedef enum {
     FPGA_FAMILY_LATTICE_MACHXO3,
 } fpga_family;
 
+/* Programming method — which backend drives this part. */
+typedef enum {
+    FPGA_PROG_NONE = 0,     /* no known method */
+    FPGA_PROG_PROXY_SPI,    /* Xilinx external SPI flash via the BSCAN proxy */
+    FPGA_PROG_SVF,          /* play a vendor-exported SVF (Lattice/Microsemi/…) */
+} fpga_prog_method;
+
 /* Caveat flags: known hardware gotchas for a part. */
 #define FPGA_CAVEAT_CCLK_VIA_STARTUP   (1u << 0)  /* CCLK not directly drivable in EXTEST */
 
@@ -53,6 +60,7 @@ typedef struct {
     const char    *proxy_bitstream;  /* path under bscan_proxies/, NULL if not yet available */
     unsigned int   caveats;          /* FPGA_CAVEAT_* flags */
     int            max_tck_khz;      /* max safe JTAG TCK for this part/board, 0 = unspecified */
+    fpga_prog_method prog;           /* programming backend; inferred when omitted */
 } fpga_target;
 
 /* Registry access. The YAML file is loaded lazily on first call to any
@@ -61,6 +69,7 @@ int                 fpga_get_target_count(void);
 const fpga_target * fpga_get_target_by_index(int index);
 const fpga_target * fpga_lookup_by_idcode(unsigned long idcode);
 const char *        fpga_family_name(fpga_family f);
+const char *        fpga_prog_method_name(fpga_prog_method m);
 
 /* Path the registry was loaded from, or NULL if nothing loaded
  * (file missing / parse error). For diagnostics. */