docs(phase4): Modbus Int64/UInt64, FOCAS fail-fast+scaling, Historian Total+dead-letter cap

docs/drivers/FOCAS.md

@@ -78,6 +78,13 @@ The driver picks its client from `Config.Backend`:
 | `wire` (default) | `WireFocasClient` | Production — pure-managed FOCAS2 over TCP |
 | `unimplemented` / `none` / `stub` | `UnimplementedFocasClientFactory` | Scaffolding a DriverInstance row before the CNC endpoint is reachable |
 
+**Fail-fast on `unimplemented` / `none` / `stub`:** a driver instance configured with any of
+these backends now **faults immediately at `InitializeAsync`** with a clear error message rather
+than reporting healthy and then failing on the first read/write/subscribe. The driver moves to
+`Faulted` state and the error is visible on the Admin UI driver-status panel. This is intentional
+— an operator who forgets to switch to `"wire"` before deploying to production sees a driver
+fault at startup, not a phantom-healthy driver that silently rejects every request.
+
 Previous backends (`fwlib`, `fwlib32`, `ipc`) have been retired along
 with `Driver.FOCAS.Host` and the Fwlib P/Invoke path. Configs that still
 reference them will throw at startup with a message pointing here.
@@ -140,11 +147,26 @@ covers `Spindle/`, `Program/` + `OperationMode/`, `Timers/`, and
 per-axis `ServoLoad` independently. Identity + `Axes/*` position reads
 (which every Fanuc CNC supports) are always emitted.
 
-Position values are scaled integers (matching FOCAS's convention). The
-managed side exposes them as `Float64` OPC UA nodes; a future
-`cnc_getfigure` integration will add per-axis decimal scaling. Until
-then, treat the raw integer as the value the CNC reports and scale on
-the client side if decimal precision matters.
+Position values (`AbsolutePosition`, `MachinePosition`, `RelativePosition`, `DistanceToGo`)
+are CNC-internal scaled integers exposed as `Float64` OPC UA nodes. The driver converts them
+to engineering units using the per-device `PositionDecimalPlaces` config field (default `0`
+= no scaling). When set to a positive integer *d*, each position value is divided by `10^d`
+before publishing, so a CNC that reports millimetres × 1000 is corrected by setting
+`PositionDecimalPlaces: 3`.
+
+```jsonc
+"Devices": [
+  {
+    "HostAddress": "focas://10.20.30.40:8193",
+    "Series": "ThirtyOne_i",
+    "PositionDecimalPlaces": 3   // 123456 → 123.456 mm
+  }
+]
+```
+
+Auto-fetching the decimal-place count via `cnc_getfigure` is deferred (wire-gated). Until
+that lands, the config field is the authoritative source — consult the MTB / machine
+parameter sheets for the correct value. Negative values are clamped to `0`.
 
 **Still user-authored**: `PARAM:6711`, `MACRO:500`, `R100` etc. — specific
 numbers whose meaning is MTB-specific. Those go under the device folder
@@ -221,10 +243,12 @@ latency spike once per cadence.
 
 | Symptom | Likely cause | Fix |
 |---------|--------------|-----|
+| Driver faults immediately at startup with "unimplemented" in the error | `Backend` is `"unimplemented"` / `"none"` / `"stub"` | Change `Backend` to `"wire"` and supply the real CNC endpoint in `Devices[]` |
 | `BadCommunicationError` on every read | CNC unreachable on TCP:8193 | Check firewall / LAN reachability; FOCAS Ethernet option must be licensed on the CNC side |
 | Every read returns `BadNotWritable` on writes | Expected — OtOpcUa is read-only against FOCAS | If you actually need writes, open a feature request — the driver's managed wire client doesn't expose the write commands |
 | `BadOutOfRange` on reads for a macro/parameter | Config address outside the declared `Series` range | Check `docs/v2/focas-version-matrix.md` — either fix the address or widen the `Series` |
 | Alarm events never fire | `AlarmProjection.Enabled` left at default (false) | Set it to `true` in the driver config |
+| Axis position values seem 1000× too large | `PositionDecimalPlaces` not set | Add `"PositionDecimalPlaces": 3` (or the MTB-specific value) to the device entry in `Devices[]` |
 
 ## Further reading