Task #138 — Modbus addressing grammar docs + e2e

Closes the docs/e2e end of the Modbus addressing line shipped across
#136-#145.

Docs:

- docs/v2/modbus-addressing.md (new) — full grammar reference.
  Region+offset (Modicon 5-digit / 6-digit / mnemonic), bit suffix,
  type codes (BOOL / I / UI / DI / UDI / LI / ULI / F / D / BCD / LBCD /
  STR<n>), all four byte-order mnemonics (ABCD / CDAB / BADC / DCBA),
  array-count semantics, family-native syntax (DL205 V/Y/C/X/SP and
  MELSEC D/M/X/Y with hex-vs-octal sub-family selection), driver-instance
  options (KeepAlive / Reconnect / IdleDisconnect, MaxCoilsPerRead and
  FC15/16 forcing, Deadband + WriteOnChangeOnly, MaxReadGap +
  CoalesceProhibited, multi-unit IPerCallHostResolver). Includes a worked
  JSON DTO example mixing AddressString + structured tag forms.

- docs/Driver.Modbus.Cli.md — appended a "v2 addressing grammar" section
  pointing users at the full reference, with quick-reference examples.

- Vendor-compatibility caveat documented: type codes and byte-order
  mnemonics were synthesised from training-era vendor docs (Wonderware
  DASMBTCP, Kepware KEPServerEX, Ignition, Matrikon, OAS) and should be
  verified against current vendor manuals before locking for production.

E2E tests (4 new AddressingGrammarTests in IntegrationTests):
- Modicon 5-digit and 6-digit forms map to identical wire offsets.
- Float32 + WordSwap (CDAB) round-trips end-to-end through the
  pymodbus simulator.
- Int16[5] array round-trips as a typed short[] surface.
- Block-read coalescing produces a wire-acceptable PDU when MaxReadGap=5
  bridges three nearby tags.

All tests skip gracefully when the pymodbus simulator at localhost:5020
is unreachable (matches the existing ModbusSimulatorFixture pattern).

Final test count across the Modbus addressing surface:
- 107 ModbusAddressing.Tests (parser + family + Modicon)
- 231 Driver.Modbus.Tests (driver, byte order, array, multi-unit, coalescing,
  protocol, subscribe, connection options)
- 110 Admin.Tests (incl. ModbusOptionsViewModel defaults pinning)
- 4 new AddressingGrammar integration tests (skip when sim down)

docs/Driver.Modbus.Cli.md

@@ -119,3 +119,31 @@ address.
 **"What's the right byte order for this family?"** → `read` with
 `--byte-order BigEndian`, then with `--byte-order WordSwap`. The one that
 gives plausible values is the correct one for that device.
+
+## v2 addressing grammar
+
+The driver accepts the industry-standard tag-address grammar so you can
+paste tag spreadsheets from Wonderware / Kepware / Ignition without
+per-row manual translation. Full reference + grammar rules:
+[`docs/v2/modbus-addressing.md`](v2/modbus-addressing.md).
+
+Quick examples:
+
+```
+40001                  HoldingRegisters[0], Int16
+400001                 same, 6-digit form
+40001:F                Float32
+40001:F:CDAB           Float32 word-swapped
+40001:STR20            20-char ASCII string
+40001:I:5              Int16[5] array (3-field shorthand)
+40001:F:CDAB:10        Float32[10] with explicit word-swap (4-field strict)
+40001.5                bit 5 of HR[0]
+HR1:DI                 Int32 via mnemonic region prefix
+C100                   Coil 100 (mnemonic, 1-based)
+V2000:F:CDAB           DL205 V-memory at PDU 1024 + Float32 + word-swap (Family=DL205)
+D100:I                 MELSEC D-register 100 (Family=MELSEC)
+```
+
+In `DriverConfig` JSON, set the per-tag `addressString` field instead of
+the structured `region` + `address` + `dataType` fields. Both styles can
+coexist within one driver instance.