Auto: focas-f4b — cnc_wrmacro + cnc_wrparam writes

Closes #269

docs/v2/implementation/focas-wire-protocol.md

@@ -19,6 +19,8 @@ Each FOCAS-equivalent call gets a stable wire-protocol command id. Ids are
 | `0x0003` | `cnc_rdmacro` | macro variable value |
 | `0x0004` | `cnc_rddiag` | diagnostic value |
 | ... | ... | ... |
+| **`0x0102`** | **`cnc_wrparam`** | **IODBPSD parameter-write packet (issue #269, plan PR F4-b)** |
+| **`0x0103`** | **`cnc_wrmacro`** | **ODBM macro-write packet (issue #269, plan PR F4-b)** |
 | `0x0F1A` | **`cnc_rdalmhistry`** | **ODBALMHIS alarm-history ring-buffer dump (issue #267, plan PR F3-a)** |
 
 ## ODBALMHIS — alarm history (`cnc_rdalmhistry`, command `0x0F1A`)
@@ -74,3 +76,81 @@ DST transitions. The .NET decoder
   unstable anyway.
 - `msg_len` overrunning the payload truncates the entry list at the
   malformed entry rather than throwing.
+
+## IODBPSD — parameter write (`cnc_wrparam`, command `0x0102`)
+
+Issue #269, plan PR F4-b. The write-side payload is the **byte-symmetric
+inverse of the `cnc_rdparam` read** — the same `IODBPSD` struct shape, and
+the .NET wire client uses the read-side decoder reversed (`EncodeParamValue`
+in `FwlibFocasClient.cs`) so the encoder/decoder are guaranteed to stay in
+lock-step.
+
+### Request
+
+| Offset | Width | Field |
+| --- | --- | --- |
+| 0 | `int16 LE` | `datano` — parameter number (e.g. `1815`) |
+| 2 | `int16 LE` | `type` — axis index (1-based; `0` = whole-CNC parameter) |
+| 4 | `length` | `data` payload — width depends on parameter type |
+
+`length` (request frame trailer, drives `data` width):
+
+| FocasDataType | `length` | Payload encoding |
+| --- | --- | --- |
+| `Byte` | `4 + 1` | one signed byte at offset 4 |
+| `Int16` | `4 + 2` | int16 LE at offset 4 |
+| `Int32` | `4 + 4` | int32 LE at offset 4 |
+
+Bit-addressed parameters (`PARAM:1815/0` form) are not supported by F4-b
+and surface as `BadNotSupported`; F4-c will land the read-modify-write
+helper alongside the PMC bit RMW path.
+
+### Response
+
+Single `int16 LE` return code per the standard FWLIB convention:
+
+- `0` → `Good`
+- `11` (`EW_PASSWD`) → **`BadUserAccessDenied`** (was `BadNotWritable`
+  pre-F4-b — see `FocasStatusMapper`). Means the parameter-write switch is
+  off or the CNC isn't in MDI mode; the F4-d unlock workflow will close
+  the loop on this from the OPC UA side.
+- Other `EW_*` codes map per
+  [`FocasStatusMapper.MapFocasReturn`](../../src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasStatusMapper.cs).
+
+## ODBM — macro write (`cnc_wrmacro`, command `0x0103`)
+
+Issue #269, plan PR F4-b. The write-side payload mirrors the
+`cnc_rdmacro` read shape: the same `(mcr_val, dec_val)` (integer +
+decimal-point count) split, but emitted from the .NET side rather than
+decoded.
+
+### Request
+
+| Offset | Width | Field |
+| --- | --- | --- |
+| 0 | `int16 LE` | `number` — macro variable number (e.g. `500`) |
+| 2 | `int16 LE` | `length` — fixed at `8` for ODBM |
+| 4 | `int32 LE` | `mcr_val` — scaled integer value |
+| 8 | `int16 LE` | `dec_val` — decimal-point count |
+
+F4-b ships **integer-only writes** (`dec_val = 0`) to match the most
+common HMI pattern; a future `WriteMacroScaled` overload will land if the
+field calls for fractional macro setpoints. Read-side decoders apply
+`mcr_val / 10^dec_val`, so a `dec_val = 0` write surfaces back as the
+integer it was emitted as.
+
+### Response
+
+Same single-int16 envelope as `cnc_wrparam`. `EW_PASSWD` is rare on macro
+writes (the gate-switch protection is parameter-specific) but the mapper
+treats both kinds identically.
+
+### Symmetry note
+
+The plan carries a "byte layout symmetry" requirement — the encoder for
+each kind is the read-side decoder reversed. Adding a new parameter type
+(e.g. `Int64` parameters, when they ship) means extending both sides in
+the same PR; the unit test
+`FocasWriteParameterTests.ParameterWrite_round_trip_stores_value_visible_to_subsequent_read`
+exercises encode → store → decode with the fake wire client and is the
+canary for symmetry regressions.