README: nvflash install instructions for Ubuntu

README.md

@@ -13,7 +13,7 @@ Reference harness (auto-cloned into `harness/` by preflight):
 - **sudo privileges** — precondition for `--preflight` package installation and for privileged checks (PCI rescan, link retrain, sysfs ROM, `dmesg`, `verify.sh`)
 - `pciutils` (`lspci`, `setpci`), `git` — installed automatically by preflight
 - NVIDIA driver with `nvidia-smi` (nvidia-open 610.xx+, see cmpunlocker) for unlock verification — preflight can install it for you (asks first)
-- `nvflash` (not packaged — download from NVIDIA or pass `--nvflash /path/to/nvflash`) for VBIOS backup and serial auto-read
+- `nvflash` (not packaged for Ubuntu — manual install, see [Installing nvflash](#installing-nvflash-manual--not-packaged-for-ubuntu) below; or pass `--nvflash /path/to/nvflash`) for VBIOS backup and serial auto-read
 - Optional: `vkmark` or `vulkan-tools` for the 5-minute VRAM stress test
 
 ## Preflight (setup step)
@@ -32,6 +32,43 @@ On Ubuntu 26.04 this:
 
 Skip installation with `--no-install` (no sudo needed at all).
 
+## Installing nvflash (manual — not packaged for Ubuntu)
+
+`nvflash` ships no `.deb` for Ubuntu/Debian. Install NVIDIA's official Linux build
+(TechPowerUp hosts it):
+
+```bash
+# 1. deps + download
+sudo apt install -y unzip
+cd ~/Downloads
+# get it from: https://www.techpowerup.com/download/nvidia-nvflash/
+# (Linux build: nvflash_5.867_linux.zip)
+
+# 2. verify integrity (SHA256 for the 5.867 linux zip)
+echo "73ab51e121129059f1377b3bf21a8001f6649d64af2471455499c803a9c80301  nvflash_5.867_linux.zip" | sha256sum -c
+
+# 3. extract + install system-wide
+unzip -o nvflash_5.867_linux.zip -d /tmp/nvflash
+sudo install -m 755 "$(find /tmp/nvflash -type f -name 'nvflash' | head -1)" /usr/local/bin/nvflash
+
+# 4. sanity check
+nvflash --version          # or: nvflash --help
+sudo nvflash --list        # must show the 170HX
+```
+
+Then re-run preflight — the tool table should show `nvflash OK`:
+
+```bash
+python3 check_cards.py --preflight
+```
+
+Notes:
+
+- The script invokes nvflash through `sudo` — cache credentials first (`sudo -v`) or run the script as root.
+- Only `--save` (read-only backup) is used; if it fails while the NVIDIA driver is bound, the script automatically falls back to the sysfs ROM method.
+- Do **not** use the AUR nvflash package (known broken), and ignore Windows `.exe` builds.
+- Alternative: point the script at a custom location with `--nvflash /path/to/nvflash`.
+
 ## Usage (one card per run)
 
 ```bash