Unified environments topology page with ElkSharp SVG layout

Replace 3 fragmented environment views (2 hardcoded stubs + tables component)
and D3.js force-directed map with a single unified topology page at
/environments/overview. The page renders an interactive SVG graph using
ElkSharp compound layout (regions as parent containers, environments as
child nodes, promotion paths as directed edges with gate labels).

Backend: new GET /api/v2/topology/layout endpoint that builds ElkGraph
from topology read model, runs ElkSharp compound layout, returns enriched
positioned nodes and routed edges.

Frontend: topology-graph.component.ts (SVG renderer with zoom/pan/select),
topology-graph-page.component.ts (filter bar + graph + detail side panel).

Deleted: environments-list-page, platform-setup-regions-environments-page,
topology-map-page, topology-regions-environments-page. Routes consolidated
from ~12 paths to 6 with backward-compat redirects.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Platform/StellaOps.Platform.WebService/Endpoints/TopologyReadModelEndpoints.cs

@@ -272,6 +272,27 @@ public static class TopologyReadModelEndpoints
         .WithSummary("List topology workflows")
         .RequireAuthorization(PlatformPolicies.TopologyRead);
 
+        topology.MapGet("/layout", async Task<IResult>(
+            HttpContext context,
+            PlatformRequestContextResolver resolver,
+            TopologyLayoutService layoutService,
+            [AsParameters] TopologyLayoutQuery query,
+            CancellationToken cancellationToken) =>
+        {
+            if (!TryResolveContext(context, resolver, out var requestContext, out var failure))
+            {
+                return failure!;
+            }
+
+            var result = await layoutService.GetLayoutAsync(
+                requestContext!, query, cancellationToken).ConfigureAwait(false);
+
+            return Results.Ok(result);
+        })
+        .WithName("GetTopologyLayoutV2")
+        .WithSummary("Get positioned topology layout for SVG rendering")
+        .RequireAuthorization(PlatformPolicies.TopologyRead);
+
         topology.MapGet("/gate-profiles", async Task<IResult>(
             HttpContext context,
             PlatformRequestContextResolver resolver,