feat(voyage)!: marketplace handoff — rename plugins/ultraplan-local to plugins/voyage [skip-docs]

Session 5 of voyage-rebrand (V6). Operator-authorized cross-plugin scope.

- git mv plugins/ultraplan-local plugins/voyage (rename detected, history preserved)
- .claude-plugin/marketplace.json: voyage entry replaces ultraplan-local
- CLAUDE.md: voyage row in plugin list, voyage in design-system consumer list
- README.md: bulk rename ultra*-local commands -> trek* commands; ultraplan-local refs -> voyage; type discriminators (type: trekbrief/trekreview); session-title pattern (voyage:<command>:<slug>); v4.0.0 release-note paragraph
- plugins/voyage/.claude-plugin/plugin.json: homepage/repository URLs point to monorepo voyage path
- plugins/voyage/verify.sh: drop URL whitelist exception (no longer needed)

Closes voyage-rebrand. bash plugins/voyage/verify.sh PASS 7/7. npm test 361/361.

plugins/voyage/lib/parsers/jaccard.mjs

@@ -0,0 +1,41 @@
+// lib/parsers/jaccard.mjs
+// Jaccard similarity for SC4 determinism floor.
+//
+// jaccard(A, B) = |A ∩ B| / |A ∪ B|
+// Inputs are arrays of strings; deduplicated internally.
+// Both empty → 1.0 (vacuously identical). One empty → 0.0.
+
+/**
+ * Compute Jaccard similarity between two string sets.
+ * @param {string[]} setA
+ * @param {string[]} setB
+ * @returns {number} similarity in [0, 1]
+ */
+export function jaccardSimilarity(setA, setB) {
+  if (!Array.isArray(setA) || !Array.isArray(setB)) {
+    throw new TypeError('jaccardSimilarity: both inputs must be arrays');
+  }
+  const a = new Set(setA);
+  const b = new Set(setB);
+  if (a.size === 0 && b.size === 0) return 1.0;
+  if (a.size === 0 || b.size === 0) return 0.0;
+
+  let intersection = 0;
+  for (const x of a) {
+    if (b.has(x)) intersection += 1;
+  }
+  const union = a.size + b.size - intersection;
+  return intersection / union;
+}
+
+/**
+ * Check whether a similarity meets a threshold.
+ * @param {number} similarity
+ * @param {number} threshold
+ * @returns {boolean}
+ */
+export function meetsThreshold(similarity, threshold) {
+  if (typeof similarity !== 'number' || typeof threshold !== 'number') return false;
+  if (!Number.isFinite(similarity) || !Number.isFinite(threshold)) return false;
+  return similarity >= threshold;
+}