♻️ refactor(toolchain): replace impact/bootgen with xsct/program_flash + add fsbl

BREAKING: Replaces obsolete impact (ISE-era) and misused bootgen with
the correct Xilinx 2018.3–2023.2 toolchain.

Toolchain changes:
- vitis_checker: impact → xsct + program_flash + bootgen detection
- flash_programmer: bootgen/impact → program_flash (erase + program + verify)
- bitstream_programmer: impact → xsct fpga/dow (primary), vivado (fallback)
- Removed duplicated _get_impact_path() — now shared from vitis_checker

Config additions:
- fsbl_elf_path: FSBL ELF required by program_flash for QSPI init
- flash_type: QSPI mode (default: qspi-x4-single)

GUI additions:
- FSBL ELF file selector in Configuration panel
- Step 2 validates FSBL ELF is selected
- Step 1 now shows xsct/program_flash/bootgen in tool check

config/default_config.yaml

@@ -19,7 +19,12 @@ bootloader_bit_path: ""
 bootloader_elf_path: ""
 
 # Flash programming (Step 2: Program & Verify Flash)
+# bootloader_bin_path is the bootloader image to program into QSPI flash
+# fsbl_elf_path is the FSBL (First Stage Boot Loader) required by program_flash
+# flash_type sets the QSPI interface mode (qspi-x4-single, qspi-x8-dual_parallel, etc.)
 bootloader_bin_path: ""
+fsbl_elf_path: ""
+flash_type: "qspi-x4-single"
 
 # Firmware upload (Step 4: Load Main Program via TFTP)
 firmware_bin_path: ""