Grid calibration: correct lever is output-priority SUB, add grid-cal-monitor

Discovered live 2026-06-25 driving an actual grid calibration: forcing a full
grid charge is done via OUTPUT PRIORITY, not voltage thresholds.
- SBU (everyday) won't grid-charge unless the bank is critically low; setting
  charger_priority=solar_and_utility alone does nothing at 52V.
- SUB (output_priority=solar_utility_battery) runs loads on grid AND charges the
  battery to full. Combined with charger_priority=solar_and_utility, grid charging
  engages (device_mode->Hybrid/Line, line_dir->input, pack current jumps to ~120A).
- Both POP/PCP set via lvx-control (all-mode-safe, atomic, no flash/USB). Revert
  POP->solar_battery_utility, PCP->solar_first when done.

The re_discharge/flash.py approach is dead (firmware NAKs stop_charge>float);
profile eg4-lp4-v2-calibration.yaml marked DEPRECATED.

- grid-cal-monitor: supervises a SUB grid charge, safety aborts (cell>3.60V/
  temp>45C), detects re-anchor (all 6 packs ->100%), auto-reverts POP+PCP (trap).
- calibration-charge skill §3 rewritten to the POP lever.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/skills/calibration-charge/SKILL.md

@@ -53,18 +53,26 @@ Record and check:
   curve to bulk 56.4 V and holds absorption on its own — exactly the clean termination
   the BMS needs to re-anchor. No flash, no risk. Just monitor §4 and verify §5; skip §3.
   This is the method to default to.
-- **Grid-assist — ADVANCED / UNVALIDATED:** the field flash.py calls
-  `stop_charge_voltage` is really `battery_re_discharge_voltage`; the firmware **NAKs
-  `0`/"Full"** (confirmed 2026-06-24 — both units rejected `BUCD480,000`). The corrected
-  lever raises it 54.0 → 56.0 so grid charges higher, but it may band-oscillate near
-  56 V instead of holding a clean absorption, so it is **not proven** to re-anchor.
-  Only use as a supervised experiment when solar can't reach full and you accept it
-  may not fully work. Needs §3.
+- **Grid-assist — for cloudy days (CORRECT lever, validated 2026-06-25):** force a full
+  grid charge by switching **output priority to SUB**, NOT by touching voltage
+  thresholds. In the everyday SBU mode the inverter won't grid-charge unless the bank is
+  critically low; SUB makes it run loads on grid AND charge the battery to full. Both
+  setters go through lvx-control (all-mode-safe, atomic, no flash). Needs §3.
+  (Do NOT use `re_discharge`/flash.py — firmware NAKs it; see memory
+  `project_lvx6048_grid_charge_lever`.)
 
-## 3. Grid-assist (ADVANCED): apply the calibration profile (USER-CONFIRMED setter change)
-Run `flash.py diff` FIRST and confirm the ONLY change is `stop_charge_voltage 54.0 ->
-56.0` before applying. If any apply NAKs, the cluster is unchanged (flash.py aborts on
-first failure) — fall back to solar-only.
+## 3. Grid-assist: enable grid charging via output priority (USER-CONFIRMED)
+Publish via lvx-control (atomic to both units, no powermon stop):
+```bash
+B=...   # broker creds from ~/.config/powermon/powermon.yaml
+mosquitto_pub ... -t solar/control/lvx6048/charger_priority -m solar_and_utility
+mosquitto_pub ... -t solar/control/lvx6048/output_priority  -m solar_utility_battery   # SUB
+```
+Confirm within ~1 min: `device_mode` -> Hybrid/Line, `line_power_direction` -> input,
+pack current jumps. Verify BOTH units match (output_source_priority) — parallel sync.
+Then run `lib/grid-cal-monitor` (detached) to drive/verify/auto-revert; it reverts
+`output_priority->solar_battery_utility` + `charger_priority->solar_first` on completion
+(trap-guaranteed). Skip §3 entirely for the solar-only method.
 Mirror to BOTH inverters (parallel cluster — mismatched settings throw fault 86).
 `flash.py apply` stops powermon for exclusive USB, so MQTT telemetry pauses briefly.
 ```bash