✨ feat: Xilinx Kit directory selector + multi-version scan

Configuration panel now has a 'Xilinx Kit:' row at the top with:
- Entry field showing current path
- [Browse] button using system directory dialog (cross-platform)
- Select root directory (e.g. /data/xilinx or C:/Xilinx)

vitis_checker.py now scans xilinx_root subdirectories:
  1. System PATH (shutil.which) — works on all platforms
  2. Legacy vitis_path/bin
  3. xilinx_root/Vitis/*/bin and Vivado/*/bin (all versions, newest first)
  4. Common install paths (Windows: C:/Xilinx, Linux: /data/xilinx, /opt/xilinx)

Multi-version: scans all version directories under Vitis/ and Vivado/,
picks the newest version for each tool.

Config: xilinx_path replaces vitis_path (backward compat kept)

src/bitstream_programmer.py

@@ -166,7 +166,10 @@ def program_bitstream(
     ps7_tcl = _find_ps7_init_tcl(config)
 
     # Try xsct first (preferred)
-    xsct = get_xsct_path(config.vitis_path)
+    xsct = get_xsct_path(
+        vitis_path=config.vitis_path if hasattr(config, 'vitis_path') else "",
+        xilinx_root=config.xilinx_path if hasattr(config, 'xilinx_path') else "",
+    )
     if xsct:
         timeout = config.step_timeouts.get("bitstream", 60)
         return _program_with_xsct(xsct, bit_path, callback, ps7_tcl, timeout)
@@ -340,7 +343,10 @@ def run_elf(
         callback("start", f"Running ELF: {elf_path.name}...")
 
     # Use xsct for CPU operations (Vivado HM cannot program PS CPU)
-    xsct = get_xsct_path(config.vitis_path)
+    xsct = get_xsct_path(
+        vitis_path=config.vitis_path if hasattr(config, 'vitis_path') else "",
+        xilinx_root=config.xilinx_path if hasattr(config, 'xilinx_path') else "",
+    )
     if not xsct:
         return BitstreamResult(
             step="elf",