feat(frontend): pure helpers for cross-subject plot interactions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/crossSubjectChart.js

@@ -0,0 +1,52 @@
+// Pure helpers for the cross-subject metrics plot interactions. No React, no I/O.
+
+// Sort a Recharts tooltip payload by numeric value descending, dropping entries
+// whose value is null/undefined. Returns a NEW array (does not mutate input).
+export function sortPayloadByValueDesc(payload) {
+  if (!Array.isArray(payload)) return [];
+  return payload
+    .filter((p) => p && p.value != null)
+    .sort((a, b) => b.value - a.value);
+}
+
+// Toggle an id's membership in a Set, returning a NEW Set.
+export function toggleInSet(set, id) {
+  const next = new Set(set);
+  if (next.has(id)) next.delete(id);
+  else next.add(id);
+  return next;
+}
+
+// Add (hidden=true) or remove (hidden=false) a list of ids from a hidden-set.
+// Returns a NEW Set.
+export function setIdsHidden(set, ids, hidden) {
+  const next = new Set(set);
+  for (const id of ids) {
+    if (hidden) next.add(id);
+    else next.delete(id);
+  }
+  return next;
+}
+
+// Tri-state for a group checkbox given which subject ids are hidden.
+// 'all' = all visible, 'none' = all hidden, 'some' = mixed.
+export function groupVisibilityState(subjectIds, hiddenSet) {
+  if (subjectIds.length === 0) return 'none';
+  const hiddenCount = subjectIds.filter((id) => hiddenSet.has(id)).length;
+  if (hiddenCount === 0) return 'all';
+  if (hiddenCount === subjectIds.length) return 'none';
+  return 'some';
+}
+
+// Group chart "lines" by their .group field, preserving first-seen group order
+// and line order within each group. Returns [{ group, subjects: [line...] }].
+export function groupLines(lines) {
+  const order = [];
+  const byGroup = new Map();
+  for (const line of lines) {
+    const g = line.group ?? '—';
+    if (!byGroup.has(g)) { byGroup.set(g, []); order.push(g); }
+    byGroup.get(g).push(line);
+  }
+  return order.map((group) => ({ group, subjects: byGroup.get(group) }));
+}