fix(#2492): split rebuild-queue DAG lines on fan-out points, not just WCC

nodeComponents() split a DAG into weakly-connected components via deps
edges, but post #2476/#2450 every agent's rebuild subgraph hangs off a
shared MetaLock node via AfterOk, so the whole meta-update cascade is one
connected component and rendered as a single wall-of-chips line.

Add a second pass (splitFanOut) that further splits a component's
topo-ordered nodes on out-degree>1 points: a node with more than one
direct dependent renders as its own one-node line, and each dependent
becomes the root of an independent line. Purely deps-structure-driven,
same as the existing WCC split - no agent-field grouping involved. A
component with no fan-out (the common single-agent case) is unaffected.

frontend/packages/dashboard/src/builds.js

@@ -178,6 +178,64 @@ function entryAgents(entry) {
   return seen.join(',');
 }
 
+// Further split one weakly-connected component's topo-ordered nodes on
+// *fan-out* points — a node with more than one direct dependent — so a
+// shared gate/lock node (e.g. MetaLock, which every agent's rebuild
+// subgraph now hangs off via AfterOk since the meta-update cascade was
+// folded into one in-DAG growth instead of separate per-agent child DAGs)
+// doesn't merge N otherwise-independent per-agent chains into one
+// wall-of-chips line. Pure
+// `deps`-structure-driven, same as the WCC split above — no `agent` field
+// involved. Rule: a node with out-degree > 1 renders as its own one-node
+// line; each of its direct dependents becomes the root of an independent
+// line, walked forward until the next fan-out point or a dead end. A
+// component with no fan-out (the common single-agent case) comes back
+// unchanged as one line.
+function splitFanOut(orderedNodes) {
+  const ids = new Set(orderedNodes.map((n) => n.id));
+  const children = new Map(orderedNodes.map((n) => [n.id, []])); // dep -> direct dependents
+  for (const n of orderedNodes) {
+    for (const d of n.deps || []) {
+      if (children.has(d)) children.get(d).push(n.id);
+    }
+  }
+  const byId = new Map(orderedNodes.map((n) => [n.id, n]));
+  const indeg = new Map(orderedNodes.map((n) => [n.id, 0]));
+  for (const n of orderedNodes) {
+    for (const d of n.deps || []) {
+      if (ids.has(d)) indeg.set(n.id, indeg.get(n.id) + 1);
+    }
+  }
+  const roots = orderedNodes.filter((n) => indeg.get(n.id) === 0).map((n) => n.id);
+
+  const lines = [];
+  const visited = new Set();
+  function walk(startId, chain) {
+    let cur = startId;
+    while (cur != null && !visited.has(cur)) {
+      visited.add(cur);
+      const kids = children.get(cur) || [];
+      if (kids.length > 1) {
+        if (chain.length) lines.push(chain);
+        lines.push([byId.get(cur)]);
+        for (const k of kids) walk(k, []);
+        return;
+      }
+      chain.push(byId.get(cur));
+      cur = kids.length === 1 ? kids[0] : null;
+    }
+    if (chain.length) lines.push(chain);
+  }
+  for (const r of roots) walk(r, []);
+  // Any leftover (shouldn't happen for a DAG reachable from its roots, but
+  // guard against a dep loop / disconnected leftover rather than dropping
+  // nodes from the display).
+  for (const n of orderedNodes) {
+    if (!visited.has(n.id)) lines.push([n]);
+  }
+  return lines.length ? lines : [orderedNodes];
+}
+
 // Split a DAG's nodes into its actual weakly-connected subgraphs, using the
 // `deps` edges the backend provides — not an inferred heuristic like
 // grouping by `n.agent`. A DAG with independent subgraphs (e.g. a
@@ -185,7 +243,9 @@ function entryAgents(entry) {
 // component per subgraph; a single connected DAG stays one component. Each
 // component's nodes come back topo-sorted (Kahn's algorithm, falling back to
 // original array order for ties) so a chain renders in actual dependency
-// order rather than raw array order.
+// order rather than raw array order. Each component is then further split
+// on fan-out points (see `splitFanOut`) so a shared gate node doesn't merge
+// independent branches into one line.
 function nodeComponents(nodes) {
   if (!nodes.length) return [];
   const byId = new Map(nodes.map((n) => [n.id, n]));
@@ -243,7 +303,7 @@ function nodeComponents(nodes) {
     for (const cn of compNodes) {
       if (remaining.has(cn.id)) ordered.push(cn);
     }
-    components.push(ordered);
+    for (const line of splitFanOut(ordered)) components.push(line);
   }
   return components;
 }