Phase 6.1 Stream A.5/A.6 — WriteIdempotent flag on DriverAttributeInfo + Modbus/S7 tag records + FlakeyDriver integration tests

Per-tag opt-in for write-retry per docs/v2/plan.md decisions #44, #45, #143.
Default is false — writes never auto-retry unless the driver author has marked
the tag as safe to replay.

Core.Abstractions:
- DriverAttributeInfo gains `bool WriteIdempotent = false` at the end of the
  positional record (back-compatible; every existing call site uses the default).

Driver.Modbus:
- ModbusTagDefinition gains `bool WriteIdempotent = false`. Safe candidates
  documented in the param XML: holding-register set-points, configuration
  registers. Unsafe: edge-triggered coils, counter-increment addresses.
- ModbusDriver.DiscoverAsync propagates t.WriteIdempotent into
  DriverAttributeInfo.WriteIdempotent.

Driver.S7:
- S7TagDefinition gains `bool WriteIdempotent = false`. Safe candidates:
  DB word/dword set-points, configuration DBs. Unsafe: M/Q bits that drive
  edge-triggered program routines.
- S7Driver.DiscoverAsync propagates the flag.

Stream A.5 integration tests (FlakeyDriverIntegrationTests, 4 new) exercise
the invoker + flaky-driver contract the plan enumerates:
- Read with 5 transient failures succeeds on the 6th attempt (RetryCount=10).
- Non-idempotent write with RetryCount=5 configured still fails on the first
  failure — no replay (decision #44 guard at the ExecuteWriteAsync surface).
- Idempotent write with 2 transient failures succeeds on the 3rd attempt.
- Two hosts on the same driver have independent breakers — dead-host trips
  its breaker but live-host's first call still succeeds.

Propagation tests:
- ModbusDriverTests: SetPoint WriteIdempotent=true flows into
  DriverAttributeInfo; PulseCoil default=false.
- S7DiscoveryAndSubscribeTests: same pattern for DBx SetPoint vs M-bit.

Full solution dotnet test: 947 passing (baseline 906, +41 net across Stream A
so far). Pre-existing Client.CLI Subscribe flake unchanged.

Stream A's remaining work (wiring CapabilityInvoker into DriverNodeManager's
OnReadValue / OnWriteValue / History / Subscribe dispatch paths) is the
server-side integration piece + needs DI wiring for the pipeline builder —
lands in the next PR on this branch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriverOptions.cs

@@ -92,6 +92,14 @@ public sealed class ModbusProbeOptions
 ///     AutomationDirect DirectLOGIC (DL205/DL260) and a few legacy families pack the first
 ///     character in the low byte instead — see <c>docs/v2/dl205.md</c> §strings.
 /// </param>
+/// <param name="WriteIdempotent">
+///     Per <c>docs/v2/plan.md</c> decisions #44, #45, #143 — flag a tag as safe to replay on
+///     write timeout / failure. Default <c>false</c>; writes do not auto-retry. Safe candidates:
+///     holding-register set-points for analog values and configuration registers where the same
+///     value can be written again without side-effects. Unsafe: coils that drive edge-triggered
+///     actions (pulse outputs), counter-increment addresses on PLCs that treat writes as deltas,
+///     any BCD / counter register where repeat-writes advance state.
+/// </param>
 public sealed record ModbusTagDefinition(
     string Name,
     ModbusRegion Region,
@@ -101,7 +109,8 @@ public sealed record ModbusTagDefinition(
     ModbusByteOrder ByteOrder = ModbusByteOrder.BigEndian,
     byte BitIndex = 0,
     ushort StringLength = 0,
-    ModbusStringByteOrder StringByteOrder = ModbusStringByteOrder.HighByteFirst);
+    ModbusStringByteOrder StringByteOrder = ModbusStringByteOrder.HighByteFirst,
+    bool WriteIdempotent = false);
 
 public enum ModbusRegion { Coils, DiscreteInputs, InputRegisters, HoldingRegisters }