Merge lane Modbus: Modbus driver remediation (#741)

Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

docs/v2/modbus-addressing.md

@@ -149,7 +149,11 @@ Beyond per-tag addressing, `ModbusDriverOptions` exposes (#139–#143):
 - `UseFC15ForSingleCoilWrites` — force FC15 (write multiple coils
   qty=1) for single-coil writes. Safety/audit PLCs may require this.
 - `UseFC16ForSingleRegisterWrites` — same for FC16 vs FC06.
-- `DisableFC23` — kill switch for FC23 (currently unused; reserved).
+- `UseFC23ForMultiRegisterWrites` (default `false`) — opt-in: a 2–121-register
+  holding write is sent as one FC23 (write + read-back). A unit answering
+  exception 01 moves to FC16 until the next re-init. Without the key the
+  wire stays FC16 (#741).
+- `DisableFC23` — hard kill switch: never send FC23, even when opted in.
 
 ### Subscribe (#141)
 - Per-tag `Deadband` — suppress sub-threshold publishes on numeric tags.
@@ -165,9 +169,13 @@ Beyond per-tag addressing, `ModbusDriverOptions` exposes (#139–#143):
   this tag in isolation regardless of `MaxReadGap`).
 
 ### Block-read coalescing (#143)
-- `MaxReadGap` (default 0 = off) — gap budget the planner is willing to
-  bridge between adjacent register tags. With `MaxReadGap=10`, three tags
-  at HR 100/102/110 collapse into one FC03 of quantity 11.
+- `MaxReadGap` — gap budget the planner is willing to bridge between
+  adjacent tags of one (unit, region). With `MaxReadGap=10`, three tags at
+  HR 100/102/110 collapse into one FC03 of quantity 11. An explicit `0` =
+  coalescing off (its historical meaning); the default `8` applies only when
+  the key is absent (#741). Coils / DiscreteInputs (gap in bits), `String`
+  scalars and same-word `BitInRegister` tags coalesce too.
+- `DisableReadCoalescing` — turn coalescing off regardless of the gap.
 
 ### Coalescing auto-recovery (#148 / #150 / #151 / #152)
 - A coalesced read that fails with a Modbus exception (write-only or
@@ -231,3 +239,43 @@ the current Kepware "Modbus Ethernet Driver Help" PDF and Ignition's
 "Modbus Addressing" user-manual page — if a critical tool's mnemonics
 have shifted, add aliases in `ModbusAddressParser.TryParseType` rather
 than asking users to rewrite spreadsheets.
+
+
+## Address strings, families and region/type rules (#741)
+
+- **One authority: `ModbusTagAddressResolution` (Driver.Modbus.Contracts).** The
+  driver's tag reader, the deploy gate (`Inspect`), the AdminUI editor model
+  (load / save / `Validate` / the hint text) and the CSV import/export all ask
+  it which part of a blob is the address; none restates the rule. Its
+  agreement is pinned by one table-driven test that runs every persisted shape
+  through every consumer (`ModbusAddressAuthorityAgreementTests`).
+- **Whenever a tag carries both `region` and `address`, those keys are its
+  address** — a classic tag, an editor-pinned tag, and a pre-#741 row that
+  carried an `addressString` beside its keys (the old driver never read the
+  string) all load from the keys, exactly as before, and a family change never
+  remaps them. Only a **string-only** blob (an `addressString` with NO
+  `region`/`address` — CSV / API authoring) is resolved through
+  `ModbusAddressParser` with the **driver's** `Family` / `MelsecSubFamily`; a
+  family that cannot parse it refuses the tag (BadNodeIdUnknown). It is never
+  served as HR[0].
+- `"addressResolved": true` is editor display / CSV state only: it never makes
+  keys authoritative, and a marker on a blob WITHOUT keys is ignored (the blob
+  is string-only) and dropped on the next editor save.
+- The AdminUI never writes default keys it did not resolve: the editor model
+  stamps `region`/`address`/type only when the loaded blob had the keys, or when
+  a field is set (hand edit, CSV address cell, Resolve). A string-only,
+  non-string-`addressString` or address-less blob keeps its keys unwritten, and
+  its CSV export leaves the address cells blank.
+- The editor's "Resolve with driver family" persists the resolved
+  `region`/`address`/type fields plus `"addressResolved": true` (the pinned
+  state). Editing a resolved field by hand in the editor drops the string and
+  the marker. A typed CSV address cell keeps them only while the string still
+  describes the cells under some family (so an unchanged export → import of a
+  pinned row is lossless); a contradicting cell drops them, and the import
+  review grid reports that column as an override.
+- Coils / DiscreteInputs carry only Bool. A bit-region tag persisted with a
+  register `dataType` (the pre-#741 editor saved `Int16`) is coerced to Bool,
+  never refused. A register region with `Bool` is refused (use
+  `BitInRegister`).
+- Exception 01/02/03 are device refusals: per-tag Bad, one Warning per tag,
+  no driver Degraded; reads and writes share one comms-fault health slot.