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/script/script.c

@@ -2779,17 +2779,20 @@ static int cmd_get_pins_list(script_ctx *ctx, char *line)
 
 const char *cmd_fpga_list_help[] = {
     "",
-    "Lists the FPGA targets known to this build (registry in modules/fpga/).",
+    "Lists the FPGA targets in the registry (loaded from fpga_registry.yaml).",
     ""};
 static int cmd_fpga_list(script_ctx *ctx, char *line)
 {
     int i, n;
     const fpga_target *t;
+    const char *src;
 
     (void)line;
 
     n = fpga_get_target_count();
-    ctx->script_printf(ctx, MSG_INFO_0, "%d FPGA target(s) registered:\n", n);
+    src = fpga_registry_source();
+    ctx->script_printf(ctx, MSG_INFO_0, "%d FPGA target(s) registered (from %s):\n",
+                       n, src ? src : "<no registry loaded>");
     for (i = 0; i < n; i++) {
         t = fpga_get_target_by_index(i);
         ctx->script_printf(ctx, MSG_NONE,