feat(frontend): Topologies page — catalog browser + builder

New authed workspace page (/topologies, in the global nav) that browses the
topology catalog and builds + visualizes a topology from a kind + roles:
- lib/api/topology.ts: zod schemas + fetchTopologyCatalog (server, authed).
- TopologyGraphView: lightweight SVG renderer (no new deps), laid out per kind
  (row for pipeline/ring, star for delegation kinds, circle otherwise).
- TopologyExplorer (client): catalog list + roles input → POST
  /api/topologies/build via the /api proxy → render the graph.
- ShellNav: add the Topologies nav item.
- e2e (p8): sign in → nav → browse catalog → build a pipeline → graph renders.

Full suite green (38); npm build + TS clean. Goes live with the next server+
frontend deploy (compare/Pareto UI to follow).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

frontend/src/components/topology/TopologyExplorer.tsx

@@ -0,0 +1,96 @@
+"use client";
+
+import { useState } from "react";
+
+import type { CatalogEntry, TopologyGraph } from "@/lib/api/topology";
+
+import { TopologyGraphView } from "./TopologyGraphView";
+
+/** Browse the topology catalog, then build + visualize a topology from a kind
+ *  and a set of roles (via the /api/topologies/build endpoint). */
+export function TopologyExplorer({ catalog }: { catalog: CatalogEntry[] }) {
+  const [kind, setKind] = useState(catalog[0]?.kind ?? "hierarchical");
+  const [roles, setRoles] = useState("coordinator, researcher, writer");
+  const [graph, setGraph] = useState<TopologyGraph | null>(null);
+  const [busy, setBusy] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  const selected = catalog.find((c) => c.kind === kind);
+
+  async function build() {
+    setBusy(true);
+    setError(null);
+    try {
+      const roleList = roles
+        .split(",")
+        .map((r) => r.trim())
+        .filter(Boolean);
+      const res = await fetch("/api/topologies/build", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ kind, roles: roleList }),
+      });
+      if (!res.ok) {
+        throw new Error(`Build failed (${res.status})`);
+      }
+      setGraph((await res.json()) as TopologyGraph);
+    } catch (e) {
+      setError(e instanceof Error ? e.message : "Build failed");
+    } finally {
+      setBusy(false);
+    }
+  }
+
+  return (
+    <div className="grid gap-6 lg:grid-cols-[240px_1fr]">
+      {/* Catalog */}
+      <div className="flex flex-col gap-0.5">
+        {catalog.map((c) => (
+          <button
+            key={c.kind}
+            type="button"
+            onClick={() => setKind(c.kind)}
+            className={`rounded-lg px-3 py-2 text-left text-sm capitalize transition-colors ${
+              kind === c.kind
+                ? "bg-surface-warm text-foreground"
+                : "text-muted-foreground hover:bg-surface-warm/50"
+            }`}
+          >
+            {c.name.replace(/_/g, " ")}
+          </button>
+        ))}
+      </div>
+
+      {/* Builder */}
+      <div className="flex flex-col gap-4">
+        {selected && (
+          <p className="text-sm text-muted-foreground">{selected.description}</p>
+        )}
+        <div className="flex flex-col gap-2 sm:flex-row sm:items-end">
+          <label className="flex-1 text-xs text-muted-foreground">
+            Roles (comma-separated)
+            <input
+              value={roles}
+              onChange={(e) => setRoles(e.target.value)}
+              className="mt-1 w-full rounded-lg border border-border bg-transparent px-3 py-2 text-sm text-foreground"
+            />
+          </label>
+          <button
+            type="button"
+            onClick={build}
+            disabled={busy}
+            className="rounded-lg bg-coral px-4 py-2 text-sm font-medium text-white transition-opacity disabled:opacity-50"
+          >
+            {busy ? "Building…" : "Build"}
+          </button>
+        </div>
+        {error && <p className="text-sm text-coral">{error}</p>}
+        {graph && (
+          <div className="rounded-2xl border border-border bg-card p-4">
+            <TopologyGraphView graph={graph} />
+          </div>
+        )}
+      </div>
+    </div>
+  );
+}

frontend/src/components/topology/TopologyGraphView.tsx

@@ -0,0 +1,80 @@
+import type { TopologyGraph } from "@/lib/api/topology";
+
+const ROW_KINDS = new Set(["pipeline", "ring"]);
+const STAR_KINDS = new Set(["hierarchical", "hub_spoke", "star_moe", "market"]);
+
+const W = 640;
+const H = 340;
+
+/** Position nodes by topology kind: a row for pipeline/ring, a center+row star
+ *  for delegation kinds, a circle otherwise. */
+function layout(kind: string, n: number): { x: number; y: number }[] {
+  const cx = W / 2;
+  const cy = H / 2;
+  if (n <= 1) return [{ x: cx, y: cy }];
+
+  if (ROW_KINDS.has(kind)) {
+    const gap = (W - 120) / (n - 1);
+    return Array.from({ length: n }, (_, i) => ({ x: 60 + i * gap, y: cy }));
+  }
+  if (STAR_KINDS.has(kind)) {
+    const pts = [{ x: cx, y: 72 }];
+    const spokes = n - 1;
+    const gap = spokes > 1 ? (W - 120) / (spokes - 1) : 0;
+    for (let i = 0; i < spokes; i++) {
+      pts.push({ x: spokes > 1 ? 60 + i * gap : cx, y: H - 72 });
+    }
+    return pts;
+  }
+  const r = Math.min(W, H) / 2 - 64;
+  return Array.from({ length: n }, (_, i) => {
+    const a = (i / n) * Math.PI * 2 - Math.PI / 2;
+    return { x: cx + r * Math.cos(a), y: cy + r * Math.sin(a) };
+  });
+}
+
+/** Renders a topology graph as a lightweight SVG (no external deps). */
+export function TopologyGraphView({ graph }: { graph: TopologyGraph }) {
+  const index = new Map(graph.nodes.map((node, i) => [node.id, i]));
+  const pos = layout(graph.kind, graph.nodes.length);
+
+  return (
+    <svg
+      viewBox={`0 0 ${W} ${H}`}
+      className="h-auto w-full"
+      role="img"
+      aria-label={`${graph.kind} topology with ${graph.nodes.length} nodes`}
+    >
+      {graph.edges.map((edge, i) => {
+        const a = pos[index.get(edge.from) ?? -1];
+        const b = pos[index.get(edge.to) ?? -1];
+        if (!a || !b) return null;
+        return (
+          <line
+            key={`${edge.from}-${edge.to}-${i}`}
+            x1={a.x}
+            y1={a.y}
+            x2={b.x}
+            y2={b.y}
+            stroke="#64748b"
+            strokeWidth={1.5}
+          />
+        );
+      })}
+      {graph.nodes.map((node, i) => {
+        const p = pos[i];
+        return (
+          <g key={node.id}>
+            <circle cx={p.x} cy={p.y} r={24} fill="#e2e8f0" stroke="#94a3b8" />
+            <text x={p.x} y={p.y + 4} textAnchor="middle" fontSize={11} fontWeight={600} fill="#0b1220">
+              {node.id}
+            </text>
+            <text x={p.x} y={p.y + 42} textAnchor="middle" fontSize={11} fill="#94a3b8">
+              {node.role}
+            </text>
+          </g>
+        );
+      })}
+    </svg>
+  );
+}