fix(digest): dense-fill topicOf with -1 sentinel + surface missed indices

Greptile P2 on PR #3247: `new Array(top.length)` creates a sparse array.
If a future injected clusterFn doesn't cover every input index,
topicOf[i] would be undefined, which then silently poisons the phase-1
aggregates (topicSize[undefined] / topicMax[undefined]) and degrades
the topic sort without any observable failure.

Fill with -1 so absence is unambiguous, then validate after clusterFn
runs and throw if any index is still -1. The outer try/catch captures
the error and returns {reps: top, topicCount: top.length, error}
matching the existing contract — primary order is preserved, no crash.

No behavior change today: singleLinkCluster's union-find guarantees
every index is covered. This just guards the invariant for future
clusterFn injections.

scripts/lib/brief-dedup.mjs

@@ -319,10 +319,19 @@ export function groupTopicsPostDedup(top, cfg, embeddingByHash, deps = {}) {
       vetoFn: null,
     });
 
-    const topicOf = new Array(top.length);
+    // Dense-fill with -1 sentinel so an incomplete clusterFn (a future
+    // injection that doesn't cover every input index) surfaces as an
+    // explicit error instead of silently poisoning the phase-1 aggregates
+    // (topicSize[undefined] / topicMax[undefined] would degrade the sort).
+    const topicOf = new Array(top.length).fill(-1);
     clusters.forEach((members, tIdx) => {
       for (const i of members) topicOf[i] = tIdx;
     });
+    for (let i = 0; i < topicOf.length; i++) {
+      if (topicOf[i] === -1) {
+        throw new Error(`topic grouping: clusterFn missed index ${i}`);
+      }
+    }
 
     const hashOf = top.map((rep) =>
       titleHashHex(normalizeForEmbedding(rep.title ?? '')),