fix(overview): one bad health response no longer stops the dashboard host (scadaproj#29)

ZbHealthReportClient caught only HttpRequestException/OperationCanceledException,
so an unknown Content-Type charset (InvalidOperationException from
ReadAsStringAsync) escaped the probe, faulted Task.WhenAll and the poller
BackgroundService, and StopHost took the dashboard down.

- client: final catch-all returns an Unreachable result for that instance
- poller: per-target Guarded probe + catch-and-log per sweep in ExecuteAsync
- tests: unknown charset (both tiers), arbitrary handler exception, sweep with
  one bad instance, and the BackgroundService survival loop
- code-reviews: Overview-001 Resolved; README regenerated

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

code-reviews/Overview/findings.md

@@ -10,7 +10,7 @@
 | Last reviewed | 2026-11-09 |
 | Reviewer | Claude |
 | Commit reviewed | `91fb2b4` |
-| Open findings | 5 |
+| Open findings | 4 |
 
 ## Summary
 
@@ -71,7 +71,7 @@ live-acceptance checks).
 | Severity | High |
 | Priority | P1 |
 | Category | Error handling & resilience |
-| Status | Open |
+| Status | Resolved |
 | Location | `ZB.MOM.WW.Overview/src/ZB.MOM.WW.Overview/Polling/ZbHealthReportClient.cs:99` (catch list `:122-134`; unguarded loop `ZB.MOM.WW.Overview/src/ZB.MOM.WW.Overview/Polling/OverviewPollerService.cs:86-95`) |
 
 **Description**
@@ -125,7 +125,7 @@ its first sweep: a crash loop. Operators lose the fleet view because of a single
 
 **Resolution**
 
-_Unresolved._
+Resolved 2026-11-09 in the commit tagged `(scadaproj#29)` — `ZbHealthReportClient.SendAsync` now ends with a catch-all that returns an Unreachable result (keeping the status code when one was received), so an unknown `charset` or any other non-shutdown exception costs only that instance. Defence in depth: `OverviewPollerService` wraps each per-target probe (`Guarded`) and catches-and-logs per sweep in `ExecuteAsync`, so neither a throwing probe nor a throwing sweep can fault the `BackgroundService`. Regression tests: `ZbHealthReportClientTests.Probe_UnknownCharset_IsUnreachableNotThrown` (both tiers), `Probe_ArbitraryHandlerException_IsUnreachableNotThrown`, `OverviewPollerServiceTests.Sweep_OneInstanceWithAnUnreadableBody_DoesNotFaultTheSweep` and `ExecuteLoop_SurvivesAnInstanceWithAnUnreadableBody` — all five fail without the fix (the loop test with the same `InvalidOperationException` from the finding); the poller tests also pass with the client fix reverted, proving the per-target guard independently.
 
 ### Overview-002 — Leader address is resolved against every instance of the application, not the group, so co-located groups steal each other's chip