fpga: load registry from yaml at runtime, not compile-time

- registry moves from the array in fpga.c to fpga_registry.yaml at the
  repo root, parsed via libyaml (pkg-config yaml-0.1); adding a part is
  now a YAML edit, no rebuild
- looked up CWD-relative (like bsdl_files/), overridable with
  $BS_FPGA_REGISTRY, loaded lazily once; public API unchanged
- fpga_list shows the source file (fpga_registry_source())
- add microsemi_igloo2/smartfusion2 and lattice_machxo2/3 families,
  ready for the non-Xilinx targets
- docs updated: CLAUDE.md, README, tutorial "add a target" walkthrough

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

modules/fpga/fpga.h

@@ -11,8 +11,10 @@
  *   - path to the BSCAN proxy bitstream
  *   - per-target caveats (known hardware gotchas)
  *
- * Adding an FPGA = one entry in fpga_registry[] + its .bsd in
- * bsdl_files/ + (optionally) its proxy .bit in bscan_proxies/.
+ * The registry is loaded at runtime from a YAML file (no longer a
+ * compile-time array). Adding an FPGA = one entry in the YAML +
+ * its .bsd in bsdl_files/ + (optionally) its proxy .bit in
+ * bscan_proxies/. See fpga_registry.yaml for the format.
  */
 
 #include "jtag_core/jtag_core.h"
@@ -22,6 +24,10 @@ typedef enum {
     FPGA_FAMILY_XILINX_7,
     FPGA_FAMILY_XILINX_US,
     FPGA_FAMILY_XILINX_USP,
+    FPGA_FAMILY_MICROSEMI_IGLOO2,
+    FPGA_FAMILY_MICROSEMI_SMARTFUSION2,
+    FPGA_FAMILY_LATTICE_MACHXO2,
+    FPGA_FAMILY_LATTICE_MACHXO3,
 } fpga_family;
 
 /* Caveat flags: known hardware gotchas for a part. */
@@ -48,10 +54,15 @@ typedef struct {
     unsigned int   caveats;          /* FPGA_CAVEAT_* flags */
 } fpga_target;
 
-/* Registry access */
+/* Registry access. The YAML file is loaded lazily on first call to any
+ * of these, and cached for the process lifetime. */
 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);
 
+/* Path the registry was loaded from, or NULL if nothing loaded
+ * (file missing / parse error). For diagnostics. */
+const char *        fpga_registry_source(void);
+
 #endif