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

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

docs/Telemetry.md

@@ -112,6 +112,23 @@ it — but it does mean the panel cannot answer "which node saw this?". Adding t
 widening the wire record, not changing the page. The one sub-instance breakdown the channel *does*
 carry is per-host connectivity, below.
 
+### Driver diagnostics ride `driver-health` (Gitea #640)
+
+A driver may implement the optional `IDriverDiagnostics` (Core.Abstractions): a pure in-memory
+`IReadOnlyDictionary<string,string> GetDiagnostics()` of operator-facing facts that are not driver
+*state* (S7's negotiated PDU size is the first; others opt in later). `DriverInstanceActor` pulls it on
+every health publish (try/caught, unguarded by the invoker like `GetHostStatuses`), bounds it
+(`DriverDiagnostics.Bound`: 16 entries, text truncated to 200 chars, ordinal key order), and carries it as
+`DriverHealthChanged.Diagnostics` -> proto `DriverHealth.has_diagnostics = 13` / `map<string,string>
+diagnostics = 14` (additive; old nodes never set them, old central ignores them) -> `HostsDriverRow.DiagnosticItems`,
+rendered as a small key/value list under the driver name on `/hosts` (nothing when empty).
+
+- **null ≠ empty**: null = driver lacks the capability; the presence flag preserves an empty bag.
+- **The diagnostics digest is part of the publish fingerprint, and must stay there** - a diagnostic fact
+  (e.g. PDU size changing on a reconnect) moves no other component, so without it the dedup swallows the
+  publish. Pinned by `DriverInstanceActorDiagnosticsTests.A_diagnostics_change_republishes_despite_otherwise_unchanged_health`
+  (stable-health stub - the shared `StubDriver` returns `UtcNow` and would pass vacuously).
+
 ### Per-host connectivity rides `driver-health` (Gitea #521)
 
 `DriverHealthChanged.HostStatuses` carries each driver's `IHostConnectivityProbe.GetHostStatuses()`

docs/drivers/S7.md

@@ -223,8 +223,10 @@ The PDU size the CPU negotiates during the connection handshake (typically 240 b
 extended PDU) is snapshotted at every (re)connect and exposed as `S7Driver.NegotiatedPduSize` /
 `S7Driver.GetDiagnostics()["S7.NegotiatedPduSize"]` (0 before the first connect and after shutdown). It is
 also written to the `S7Driver connected` / `reconnected` log lines and to the AdminUI Test Connect
-message (`S7 connected (CPU S71500, PDU 240 bytes)`). `DriverHealth` has no diagnostics bag in this
-codebase, so the value is not yet on the `/hosts` health snapshot — adding one is a Core change.
+message (`S7 connected (CPU S71500, PDU 240 bytes)`). S7 also implements the optional
+`IDriverDiagnostics` capability (explicitly, string-valued: `S7.NegotiatedPduSize` = `"480"`, omitted while 0 =
+not negotiated yet), so the value rides the driver-health snapshot and shows under the driver name on the
+AdminUI `/hosts` page (Gitea #640). See `docs/Telemetry.md` ("Driver diagnostics ride `driver-health`").
 
 ## Capability surface