VBIOS backup: precise failure diagnosis plus opt-in unload retry

save_bios now distinguishes nvflash driver-loaded refusal from a
missing sysfs rom node (ROM BAR not exposed) vs enable denial,
and appends host lockdown/Secure Boot state to the note. New
opt-in last resort: unload NVIDIA modules, retry nvflash --save,
always reload (y/N prompt default n, or --bios-unload). Docs
(flow, flags, safety, AGENTS.md invariant) updated to match.

README.md

@@ -85,7 +85,7 @@ Flow of a normal run:
 
 1. Waits for a CMP 170HX on the bus — prompts you to insert/reseat the card, Enter triggers a PCI rescan (`q` quits)
 2. Folder `cards/<GPU-UUID>/` from the auto-read GPU UUID (`nvidia-smi`); physical serial probed from nvflash and saved to `serial.txt` (`N/A` when the card exposes none) — never prompts
-3. VBIOS backup → `cards/<GPU-UUID>/bios.rom` (nvflash, sysfs ROM fallback)
+3. VBIOS backup → `cards/<GPU-UUID>/bios.rom` (nvflash, sysfs ROM fallback). On failure the script reports the precise cause (driver-loaded vs missing rom node) and offers an opt-in driver unload → save → reload retry (`Unload NVIDIA driver and retry nvflash --save now? (y/N)`, default **n**; `--bios-unload` runs it without asking)
 4. PCIe checks: link speed/width, AER counters, full config-space dump
 5. Link retrain test + full bus rescan test (device must survive both)
 6. Kernel log scrape (`dmesg`/`journalctl`) for this BDF: Xid / AER / NVRM lines
@@ -136,12 +136,14 @@ cards/
 | `--vulkan ask\|always\|never` | VRAM test policy (default: ask, last step) |
 | `--unlock` | run the cmpunlocker `install.sh` step without prompting (default: ask y/N) |
 | `--skip-bios` | skip VBIOS backup |
+| `--bios-unload` | unload NVIDIA driver to retry `nvflash --save` without prompting (console may freeze briefly, driver is reloaded after) |
 | `--no-install` | never install packages (skips the sudo precondition) |
 | `--no-clone` | don't clone the harness repos |
 
 ## Safety
 
-This tool **reads** card state by default. The one step that changes the card is **opt-in**:
+This tool **reads** card state by default. Two steps can change host state, both **opt-in**:
 
 - **Unlock install (opt-in, explicit confirmation)** — runs only if you answer `y` to `Run cmpunlocker install.sh now? (y/N)` (default `n`) or pass `--unlock`. It executes `install.sh` **from the harness clone** (`harness/cmpunlocker/`, refreshed first with `git pull --ff-only`, no extra installer flags), needs root (sudo credentials are cached first), streams its output live, and saves the full log to `cards/<GPU-UUID>/install_unlocker.log`. It is skipped entirely when memory is already unlocked, when the harness is not cloned, and in `--demo` mode. A successful install replaces kernel modules — the step then skips VRAM stress and asks you to cold reboot and re-run.
+- **VBIOS unload-retry (opt-in, explicit confirmation)** — offered only after both backup methods fail with the driver loaded: unloads the NVIDIA modules, retries `nvflash --save`, then reloads the modules (local console may freeze briefly). Triggered by `y` or `--bios-unload`; skipped in `--demo` mode.
 - **Everything else remains read-only**: `verify.sh` runs without installing anything, no modules are patched and no firmware is flashed (nvflash is only ever used with `--save`). The only other writes are local files (reports, logs), PCI link retrain/rescan sysfs triggers, and package installation during `--preflight` (explicitly gated behind the sudo precondition).