docs: update pn51 stability with third freeze and conclusion

pn02 crashed again after ~2d21h uptime despite all mitigations
(amdgpu blacklist, max_cstate=1, NMI watchdog, rasdaemon).
NMI watchdog didn't fire and rasdaemon recorded nothing,
confirming hard lockup below NMI level. Unit is unreliable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/pn51-stability.md

@@ -103,25 +103,57 @@ These appear on both units and can be ignored:
 - `snd_hda_intel: no codecs found` — no audio device connected, headless server
 - `ata2.00: supports DRM functions and may not be fully accessible` — Samsung SSD DRM quirk (pn02 only)
 
+### 2026-02-23: processor.max_cstate=1 and Proxmox Forums
+
+- Found a thread on the Proxmox forums about PN51 units with similar freeze issues
+  - Many users reporting identical symptoms — random hard freezes, no log evidence
+  - No conclusive fix. Some have frequent freezes, others only a few times a month
+  - Some reported BIOS updates helped, but results inconsistent
+- Added `processor.max_cstate=1` kernel parameter to pn02 — limits CPU to C1 halt state, preventing deep C-state sleep transitions that may trigger freezes on AMD mobile chips
+- Also applied: amdgpu blacklist, panic=10, nmi_watchdog=1, softlockup/hardlockup panic, rasdaemon
+
+### 2026-02-23: logind D-Bus Deadlock (pn02)
+
+- node-exporter alert fired — but host was NOT frozen
+- logind was running (PID 871) but deadlocked on D-Bus — not responding to `org.freedesktop.login1` requests
+- Every node-exporter scrape blocked for 25s waiting for logind, causing scrape timeouts
+- Likely related to amdgpu blacklist — no DRM device means no graphical seat, logind may have deadlocked during seat enumeration at boot
+- Fix: `systemctl restart systemd-logind` + `systemctl restart prometheus-node-exporter`
+- After restart, logind responded normally and reported seat0
+
+### 2026-02-27: pn02 Third Freeze
+
+- pn02 crashed again after ~2 days 21 hours uptime (longest run so far)
+- Evidence of crash:
+  - Journal file corrupted: `system.journal corrupted or uncleanly shut down`
+  - Boot partition fsck: `Dirty bit is set. Fs was not properly unmounted`
+  - No orderly shutdown logs from previous boot
+  - No auto-upgrade triggered
+- **NMI watchdog did NOT fire** — no kernel panic logged. This is a true hard lockup below NMI level
+- **rasdaemon recorded nothing** — no MCE, AER, or memory errors in the sqlite database
+- **Positive**: The system auto-rebooted this time (likely hardware watchdog), unlike previous freezes that required manual power cycle
+- `processor.max_cstate=1` may have extended uptime (2d21h vs previous 1h and 5.5h) but did not prevent the freeze
+
+## Conclusion
+
+**pn02 is unreliable.** After exhausting mitigations (fTPM disabled, BIOS updated, WiFi/BT disabled, ErP disabled, amdgpu blacklisted, processor.max_cstate=1, NMI watchdog, rasdaemon), the unit still hard-freezes every few days. The freeze occurs below NMI level with zero diagnostic output, pointing to a board-level hardware defect. The unit is not suitable for hypervisor use or any workload requiring reliability.
+
+**pn01 remains stable** so far but has historically crashed as well, just less frequently. Continuing to monitor.
+
 ## Next Steps
 
-- Monitor pn02 with amdgpu blacklisted — if stable, try the less impactful `amdgpu.runpm=0 amdgpu.dpm=0` kernel params instead
-- If pn02 still freezes without amdgpu:
-  - Swap RAM sticks between units to check if instability follows the RAM
-  - Run memtest86 for 24h+ on each stick individually
-  - Try a different kernel (LTS 6.6 or latest unstable)
-  - Try disabling SMT (`nosmt` kernel param)
-- If nothing helps, likely a hardware defect on pn02's board
-- pn01 continues to be stable — keep monitoring
-- Once stable: add second RAM stick back to pn02, reinstall with NVMe
-- Evaluate for Incus hypervisor use (see `nixos-hypervisor.md`)
+- **pn02**: Consider scrapping or repurposing for non-critical workloads that tolerate random reboots (auto-recovery via hardware watchdog is now working)
+- **pn01**: Continue monitoring. If it remains stable long-term, may still be viable for light workloads
+- If pn01 eventually crashes, apply the same mitigations (amdgpu blacklist, max_cstate=1) to see if they help
+- For the Incus hypervisor plan: likely need different hardware. Evaluating GMKtec G3 (Intel) as an alternative. Note: mixed Intel/AMD cluster complicates live migration
 
 ## Diagnostics and Auto-Recovery (pn02)
 
 Currently deployed on pn02:
 
 ```nix
-boot.kernelParams = [ "panic=10" "nmi_watchdog=1" ];
+boot.blacklistedKernelModules = [ "amdgpu" ];
+boot.kernelParams = [ "panic=10" "nmi_watchdog=1" "processor.max_cstate=1" ];
 boot.kernel.sysctl."kernel.softlockup_panic" = 1;
 boot.kernel.sysctl."kernel.hardlockup_panic" = 1;
 hardware.rasdaemon.enable = true;