feat(overview): poller, leader aggregation and the dashboard UI

Tasks 3.4-3.6 of the overview-dashboard plan.

Polling (3.4/3.5):
- OverviewPollerService: one timer at the fastest configured cadence, each
  target polled when its own interval is due, fan-out per sweep, no retries
  (the next tick is the retry, so damping stays the only place that decides
  whether a failure is worth showing).
- OverviewSnapshotStore: atomic swap of a complete immutable graph plus a
  change event. Pages read it and never probe — the cache-first requirement.
- LeaderResolver: per-group leader from akka-cluster data, with the
  split-brain flag scoped to members that are actually answering, so an
  ordinary failover is not misread as a disagreement.

UI (3.6): ThemeShell + StatusPill/TechCard composition against the mockup;
instance cards carry the status stripe (dashed for Unreachable), the check
detail list and the raw ready/active signal line.

Two defects found by verification rather than by review:

- The active tier answers with a status code and an EMPTY body, but the client
  demanded parseable JSON on every probe — so a healthy pair rendered as
  "role unknown" instead of Active/Standby. Split into ProbeAsync (ready tier,
  body is the payload) and ProbeStatusAsync (active tier, code is the answer).
  Caught by a live smoke run against two fake health endpoints; the poller
  tests now serve an empty body so they hold the fix.

- SweepAsync published even when cancellation was already requested, leaving a
  half-probed registry as the store's final state on shutdown. It now checks
  the token itself rather than relying on the transport to throw.

Also: app.UseAntiforgery() is required despite the anonymous-by-design
pipeline — AddRazorComponents stamps antiforgery metadata unconditionally and
the endpoint middleware hard-fails without it (every page was 500). Chosen
over DisableAntiforgery() so a future form is protected by default.

147 tests, 0 warnings. Live-verified end to end against fake endpoints:
Up/Unreachable/Active/Standby, leader chip, cluster-data line, KPI counts.

docs/plans/2026-07-22-overview-dashboard-impl-plan.md.tasks.json

@@ -104,7 +104,7 @@
     {
       "id": "14",
       "subject": "3.4 Overview: poller + snapshot store",
-      "status": "pending",
+      "status": "completed",
       "blockedBy": [
         "12",
         "13"
@@ -113,7 +113,7 @@
     {
       "id": "15",
       "subject": "3.5 Overview: leader aggregation",
-      "status": "pending",
+      "status": "completed",
       "blockedBy": [
         "13"
       ]
@@ -121,7 +121,7 @@
     {
       "id": "16",
       "subject": "3.6 Overview: UI (ThemeShell, Overview page, InstanceCard)",
-      "status": "pending",
+      "status": "completed",
       "blockedBy": [
         "14",
         "15"
@@ -170,14 +170,19 @@
       "Phase 1 DoD rig curls (site :8084 ready/active split) \u2014 live docker rig, folded into Phase 4."
     ],
     "branches": {
-      "scadaproj": "feat/health-0.2.0-cluster-data (80668a0) -> feat/overview-dashboard (d0110cf), stacked",
+      "scadaproj": "feat/health-0.2.0-cluster-data (80668a0) -> feat/overview-dashboard, stacked",
       "ScadaBridge": "feat/site-node-health (commit 47850c0f)",
       "OtOpcUa": "feat/health-0.2.0-bump (commit 2e515c34, made in an isolated git worktree \u2014 feat/mesh-phase6 working tree untouched)",
       "HistorianGateway": "chore/health-0.2.0-bump (commit 51f0c2b)",
       "MxAccessGateway": "chore/health-0.2.0-bump (commit 2d54ace)"
     },
     "front_loaded": [
-      "Validator / health-client / status-derivation tests from task 3.8 were written with tasks 3.2-3.3 so each batch is verified; 3.8 covers the remainder (poller, leader, store, bUnit, boot)."
+      "Validator / health-client / status-derivation tests from task 3.8 were written with tasks 3.2-3.3 so each batch is verified; 3.8 covers the remainder (poller, leader, store, bUnit, boot).",
+      "Poller / snapshot-store / leader-resolver / bUnit InstanceCard + Overview page tests were written with tasks 3.4-3.6; task 3.8 now covers only the WebApplicationFactory boot tests."
+    ],
+    "deviations": [
+      "Task 3.6 said 'no antiforgery'. AddRazorComponents stamps antiforgery metadata on every component endpoint unconditionally and the endpoint middleware hard-fails without a matching middleware, so every page returned 500. Resolved with app.UseAntiforgery() (inert here, no forms) rather than MapRazorComponents(...).DisableAntiforgery(), which would silently expose the first form anyone adds. No authentication was added.",
+      "Bootstrap IS vendored (copied from HistorianGateway, v5.3.3). Not optional: ThemeShell and TechButton are built on Bootstrap utility classes, so the shared kit does not render without it."
     ]
   }
 }