perf(group/manifest): parallelize and memoize symbol resolution

Previous implementation ran 2N sequential Cypher round-trips per
manifest (one for provider side, one for consumer, awaited in-order
per link). For manifests with tens of links this dominated syncGroup
latency in groups with many declared cross-repo contracts.

Changes:
- Resolve provider + consumer in parallel per link (Promise.all).
- Resolve all links in parallel (outer Promise.all over links.map).
  Each repo's executor pool is independent, so cross-repo fan-out
  scales with the number of distinct repos in the manifest.
- Memoize by (repo, type, contract). Manifests frequently declare
  the same contract from both directions or across sibling groups,
  so duplicate triples now hit the DB once instead of 2× per link.

Correctness:
- resolveSymbol is a pure LIMIT 1 read, so caching + concurrent
  invocation is safe.
- Iteration order over links is preserved in the final
  contracts / crossLinks arrays — result shape is identical.

Test:
- New test asserts that two links sharing (repo, type, contract)
  produce exactly one DB call per distinct repo-tuple.

gitnexus/src/core/group/extractors/manifest-extractor.ts

@@ -79,17 +79,50 @@ export class ManifestExtractor {
     links: GroupManifestLink[],
     dbExecutors?: Map<string, CypherExecutor>,
   ): Promise<ManifestExtractResult> {
+    // Resolve all (repo, link) pairs in parallel. The previous sequential
+    // await-per-link produced 2N round-trips; parallel resolution uses the
+    // per-repo executor pool directly and scales linearly with manifest size.
+    //
+    // Memoization: a manifest can list the same contract multiple times
+    // (e.g. a consumer and provider declaration, or cross-referenced groups).
+    // Key on (repo, type, contract) — the canonical input to the Cypher
+    // query — so duplicate links resolve to one DB hit.
+    type ResolvedSymbol = { filePath: string; name: string; uid: string } | null;
+    const resolveCache = new Map<string, Promise<ResolvedSymbol>>();
+    const resolveOnce = (repo: string, link: GroupManifestLink): Promise<ResolvedSymbol> => {
+      const key = `${repo}\u0000${link.type}\u0000${link.contract}`;
+      let pending = resolveCache.get(key);
+      if (!pending) {
+        pending = this.resolveSymbol(repo, link, dbExecutors);
+        resolveCache.set(key, pending);
+      }
+      return pending;
+    };
+
+    const perLink = await Promise.all(
+      links.map(async (link) => {
+        const contractId = this.buildContractId(link.type, link.contract);
+        const providerRepo = link.role === 'provider' ? link.from : link.to;
+        const consumerRepo = link.role === 'provider' ? link.to : link.from;
+        const [providerSymbol, consumerSymbol] = await Promise.all([
+          resolveOnce(providerRepo, link),
+          resolveOnce(consumerRepo, link),
+        ]);
+        return { link, contractId, providerRepo, consumerRepo, providerSymbol, consumerSymbol };
+      }),
+    );
+
     const contracts: StoredContract[] = [];
     const crossLinks: CrossLink[] = [];
 
-    for (const link of links) {
-      const contractId = this.buildContractId(link.type, link.contract);
-
-      const providerRepo = link.role === 'provider' ? link.from : link.to;
-      const consumerRepo = link.role === 'provider' ? link.to : link.from;
-
-      const providerSymbol = await this.resolveSymbol(providerRepo, link, dbExecutors);
-      const consumerSymbol = await this.resolveSymbol(consumerRepo, link, dbExecutors);
+    for (const {
+      link,
+      contractId,
+      providerRepo,
+      consumerRepo,
+      providerSymbol,
+      consumerSymbol,
+    } of perLink) {
       const providerRef = providerSymbol || { filePath: '', name: link.contract };
       const consumerRef = consumerSymbol || { filePath: '', name: link.contract };
       // When the resolver finds a real graph symbol we keep its uid, otherwise

gitnexus/test/unit/group/manifest-extractor.test.ts

@@ -583,4 +583,34 @@ describe('ManifestExtractor', () => {
     expect(result.contracts).toHaveLength(0);
     expect(result.crossLinks).toHaveLength(0);
   });
+
+  it('memoizes repeated (repo, type, contract) resolutions so each tuple hits the DB once', async () => {
+    const calls: Array<{ repo: string; cypher: string }> = [];
+    const execFor = (repo: string) => async (cypher: string) => {
+      calls.push({ repo, cypher });
+      return [{ uid: `uid::${repo}`, name: 'handler', filePath: 'src/h.ts' }];
+    };
+
+    const dbExecutors = new Map<string, (c: string) => Promise<Record<string, unknown>[]>>([
+      ['svc/a', execFor('svc/a')],
+      ['svc/b', execFor('svc/b')],
+    ]);
+
+    // Two links declare the same (repo, type, contract) triple on each side,
+    // so naive sequential resolution would run 4 queries; memoization collapses
+    // to 2 (one per distinct repo tuple).
+    const link: GroupManifestLink = {
+      from: 'svc/b',
+      to: 'svc/a',
+      type: 'http',
+      contract: 'GET::/api/orders',
+      role: 'consumer',
+    };
+
+    await extractor.extractFromManifest([link, { ...link }], dbExecutors);
+
+    // One resolution per distinct (repo, type, contract) — not per (link × side).
+    expect(calls).toHaveLength(2);
+    expect(new Set(calls.map((c) => c.repo))).toEqual(new Set(['svc/a', 'svc/b']));
+  });
 });