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>

CLAUDE.md

@@ -89,8 +89,14 @@ derived from the BSDL alone:
 - `proxy_bitstream_path` — path to the BSCAN proxy `.bit` for this part
 - `caveats` — flags for known hardware gotchas (e.g. CCLK via STARTUPE3)
 
-Registry is a compile-time array. Adding a part = one entry + its
-`.bsd` in `bsdl_files/` + its proxy `.bit` in `bscan_proxies/`.
+Registry is a **runtime YAML file** (`fpga_registry.yaml` at the repo
+root), parsed via libyaml — no longer a compile-time array. It is looked
+up CWD-relative (like `bsdl_files/`), overridable with
+`$BS_FPGA_REGISTRY`, and loaded lazily on first access. Adding a part =
+one YAML entry + its `.bsd` in `bsdl_files/` + (optionally) its proxy
+`.bit` in `bscan_proxies/` — no rebuild. The `family` field accepts
+`xilinx_*`, `microsemi_igloo2/smartfusion2`, `lattice_machxo2/3`
+(enum in `fpga.h`).
 
 ### Digilent SMT2 modules need libdjtg, not raw MPSSE
 
@@ -249,6 +255,11 @@ mkdir build && cd build && cmake .. && make
 ./bs/bs           # interactive REPL
 ```
 
+Build needs **libyaml** (pkg-config `yaml-0.1`; Arch `libyaml`, Debian
+`libyaml-dev`) — the FPGA registry is parsed from `fpga_registry.yaml`
+at runtime. Run `bs` from the repo root so it finds that file (and
+`bsdl_files/`, `bscan_proxies/`), or point `$BS_FPGA_REGISTRY` at it.
+
 The Digilent SMT2 backend is built by default on UNIX (disable with
 `-DBS_ENABLE_DIGILENT=OFF`). To actually use such a probe, install the
 Adept Runtime system-wide (provides `libdjtg.so` + `libdmgr.so`).