feat(hosts): driver diagnostics channel on /hosts, S7 negotiated PDU size first (#640)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

CLAUDE.md

@@ -1113,8 +1113,9 @@ host logger factory (wired in `DriverFactoryBootstrap`; the FOCAS and TwinCAT fa
 - **S7 (#632/#638):** STRING/WSTRING/CHAR/WCHAR and DTL/DATE/TOD/TIME/S5TIME codecs (`S7ValueCodecs`; TOD/TIME/S5TIME
   surface as Int32 ms); an undecodable buffer or unrepresentable write is `BadOutOfRange` with no health impact.
   Optional `localTsap`/`remoteTsap`/`connectionType` override the slot TSAP (`remoteTsap` ⊕ `connectionType`). Test
-  Connect reads 2 bytes of `MB0` so a PUT/GET-disabled CPU fails the probe. The negotiated PDU size is logged and on
-  `S7Driver.GetDiagnostics()` only — there is no driver-diagnostics surface on `/hosts`.
+  Connect reads 2 bytes of `MB0` so a PUT/GET-disabled CPU fails the probe. The negotiated PDU size is logged, on
+  `S7Driver.GetDiagnostics()`, and (#640) on `/hosts` via the optional `IDriverDiagnostics` bag (string map on
+  `DriverHealthChanged.Diagnostics` / proto `has_diagnostics`+`diagnostics`; its digest is in the publish fingerprint).
 - **TwinCAT (#637):** TIME/TOD publish Float64 ms, DATE/DT a UTC DateTime; a tag authored `UDInt` keeps the raw counter
   (and keeps historizing — DateTime is not historized).
 - **AbLegacy (#636):** `AbLegacyAddress` (in `AbLegacy.Contracts`) is family-aware: PLC-5 `I:`/`O:` indices are OCTAL