Docs: reflect this session's findings across the repo

- top-level README.md (new): system overview, subsystem map, skills pointer,
  notable findings.
- eg4battery README/NOTES: 3 -> 6 packs (pack 6 oddball 0x01/115200); SoC drift
  + calibration section; closed-loop comms evaluated and rejected (loses per-pack
  telemetry, no native protocol, doesn't fix drift); how to force a grid charge
  via output-priority SUB.
- LVX6048 README: closed-loop pending item -> resolved decision; new "SoC
  calibration & known firmware quirks" section (POP single-digit/POP01, MCHGC
  charge-lock, re_discharge=re-discharge can't exceed float, PIRI lag, powermon
  adhoc wedge); skills pointer.
- lvx-control README: POP encoding fix, POP crc-but-applies quirk, verify-by-
  behavior, grid-charge-via-SUB usage.
- troubleshoot-inverter skill: corrected the stale "dead string per inverter"
  claim — both strings healthy; low PV is tilt/heat/shade/curtailment.

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

LVX6048/README.md

@@ -196,9 +196,42 @@ then restart the three services.
   PF) are implemented in `lvx-flash/flash.py` for offline use, but aren't
   exposed as HA button/select entities. Deferred until monitoring has been
   stable for at least a week.
-- **Closed-loop BMS comms.** Currently open-loop — inverters estimate SoC
-  from voltage, batteries don't push real-time SoC / charge limits to the
-  inverter. Closed-loop would give better SoC accuracy and dynamic CC
-  tapering near full. Path is the dedicated CAN port on the master pack →
-  inverter BMS port (separate cable from the inter-pack daisy-chain).
-  Deferred.
+- **Closed-loop BMS comms — evaluated 2026-06, NOT pursuing.** Stays open-loop.
+  Closed-loop would need the LP4V2 to emulate Pylontech-CAN (no native EG4
+  support on the LVX BMS port) and would require the inter-pack daisy-chain that
+  silences slave packs' RS485 ports — i.e. trade away the per-pack/per-cell
+  telemetry that's our best diagnostic. And it wouldn't fix SoC accuracy: it just
+  forwards the BMS's own drifted SoC. The cure for drift is a periodic full charge
+  (see below), done open-loop. See `../eg4battery/NOTES.md` §"Closed-loop BMS comms".
+
+## SoC calibration & known firmware quirks
+
+**SoC drift fix.** The EG4 pack SoC counters drift because the conservative profile
+rarely drives a true full charge. A periodic **full charge to absorption** re-anchors
+every pack to 100%. Automated by the `calibration-charge` skill —
+[`../.claude/skills/calibration-charge/`](../.claude/skills/calibration-charge/):
+solar-only on a clear day, or grid-assisted via **output-priority SUB** on a cloudy
+day (`../.claude/skills/lib/grid-cal-monitor`, which safety-monitors and auto-reverts).
+
+**LVX6048 firmware quirks** (learned the hard way; baked into the tooling):
+- **PI18 `POP` is single-digit.** `solar_battery_utility` encodes as `POP1`, NOT
+  `POP01` — the inverter *silently rejects* the malformed `POP01` (no error on the
+  result topic). Fixed in `lvx-control` + `lvx-flash/flash.py` `POP_MAP`. A `POP` set
+  returns "crc check fails" on the result topic but still applies; `PCP` returns "Succeeded".
+- **`MCHGC`/`MUCHGC` are locked while charging** — a charge-current change NAKs whenever
+  `mppt1_charger_status=charging` (even though `device_mode` reads `Battery`). Set them
+  only in a pre-charge idle window.
+- **`stop_charge_voltage` is really `battery_re_discharge_voltage`** and can't exceed
+  `float`; firmware NAKs `0`/"Full". Don't use it to force a grid charge — use POP=SUB.
+- **PIRI readback lags ~5 min**, so verify a setter by *behavior*
+  (`line_power_direction`, `device_mode`), not the readback entity.
+- **powermon's adhoc queue can wedge** (commands stop landing on the result topic);
+  `sudo systemctl restart powermon.service powermon2.service` clears it.
+
+## Monitoring & troubleshooting skills
+
+Agent-runnable skills for this install live in
+[`../.claude/skills/`](../.claude/skills/) (see `REFERENCE.md` there for the system
+map): `solar-health-check`, `troubleshoot-inverter`, `troubleshoot-battery`,
+`power-usage`, and `calibration-charge`, plus helpers `lib/solar-snapshot` (live MQTT)
+and `lib/ha-history` (HA recorder lookback).