feat(communities): seed Leiden RNG for deterministic community detection

The vendored Leiden algorithm defaults to Math.random for tie-breaking
and randomized walks, which produces non-deterministic community
assignments and modularity values across runs on the same graph.

Pass a seeded mulberry32 RNG (LEIDEN_SEED=0xC0DE) so:
- The same graph always produces the same partition
- Modularity values are reproducible
- Equivalence tests for incremental indexing can compare community
  assignments byte-for-byte

This is foundational for the upcoming incremental-indexing feature
(see docs/superpowers/specs/2026-05-10-incremental-indexing-design.md)
where the correctness contract is incremental output ≡ full rebuild
output.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gitnexus/src/core/ingestion/community-processor.ts

@@ -41,6 +41,24 @@ interface LeidenDetailedResult {
   modularity: number;
 }
 
+/**
+ * Deterministic PRNG (mulberry32) seed for the vendored Leiden algorithm.
+ * Vendored Leiden defaults `rng: Math.random`, which makes community
+ * assignment non-deterministic across runs. Passing a seeded RNG gives us
+ * reproducible community/modularity output, which is required for the
+ * incremental-indexing equivalence test (incremental ≡ full rebuild).
+ */
+const LEIDEN_SEED = 0xc0de;
+function createSeededRng(seed: number): () => number {
+  let s = seed >>> 0;
+  return () => {
+    s = (s + 0x6d2b79f5) >>> 0;
+    let t = Math.imul(s ^ (s >>> 15), 1 | s);
+    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
+    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+  };
+}
+
 // ============================================================================
 // TYPES
 // ============================================================================
@@ -150,6 +168,7 @@ export const processCommunities = async (
         leiden.detailed(graph, {
           resolution: isLarge ? 2.0 : 1.0,
           maxIterations: isLarge ? 3 : 0,
+          rng: createSeededRng(LEIDEN_SEED),
         }),
       ),
       new Promise<never>((_, reject) =>