Auto: s7-c1 — surface negotiated PDU size via DriverHealth.Diagnostics

Closes #294

docs/drivers/S7-Test-Fixture.md

@@ -44,6 +44,10 @@ The driver ctor change that made this possible:
   bool-with-bit in one batch call; proves typed decode per S7DataType
 - `S7_1500SmokeTests.Driver_write_then_read_round_trip_on_scratch_word` —
   `DB1.DBW100` write → read-back; proves write path + buffer visibility
+- `S7_1500DiagnosticsTests.Driver_exposes_negotiated_pdu_size_post_init` —
+  asserts `DriverHealth.Diagnostics["S7.NegotiatedPduSize"]` is non-zero
+  after `InitializeAsync`; proves the negotiated PDU size surfaces in
+  driver health (Snap7 fixture pins this at 240 bytes — see fixture README)
 
 ### Unit
 

docs/v2/s7.md

@@ -548,6 +548,31 @@ under the standard `<DriverType>.<Counter>` naming convention:
 Observe via the `driver-diagnostics` RPC (`/api/v2/drivers/{id}/diagnostics`)
 or the Admin UI's per-driver dashboard.
 
+### Diagnostics surfacing
+
+Beyond the coalescing counters above, the S7 driver also surfaces the
+**negotiated PDU size** captured during the COTP/S7comm handshake under the
+same `<DriverType>.<Counter>` naming convention:
+
+- `S7.NegotiatedPduSize` — the PDU envelope size advertised by the CPU
+  during `Plc.OpenAsync`. Default S7-1500 CPUs negotiate **240 bytes**;
+  CPUs running the extended PDU advertise **480 or 960 bytes**. The value
+  is `0` before the first successful connect and is reset to `0` on
+  driver shutdown so an operator inspecting the Admin UI dashboard can
+  immediately tell whether the driver is currently online.
+
+Together these counters answer the most common operator questions about
+S7 driver health without reaching for a Wireshark capture:
+
+- "Is the driver actually connected?" → `S7.NegotiatedPduSize > 0`
+- "Is coalescing working?" → `S7.TotalBlockReads` climbing while
+  `S7.TotalMultiVarBatches` stays flat
+- "Why is throughput poor?" → `S7.NegotiatedPduSize` is 240 instead of 960
+  (operator can switch the CPU to extended PDU if the project allows)
+
+The values render alongside Modbus / OPC UA Client metrics in the Admin
+UI driver-diagnostics panel — same RPC, same dashboard row layout.
+
 ## References
 
 1. Siemens Industry Online Support, *Modbus/TCP Communication between SIMATIC S7-1500 / S7-1200 and Modbus/TCP Controllers with Instructions `MB_CLIENT` and `MB_SERVER`*, Entry ID 102020340, V6 (Feb 2021). https://cache.industry.siemens.com/dl/files/340/102020340/att_118119/v6/net_modbus_tcp_s7-1500_s7-1200_en.pdf