fix: skip 75ms retry when store is empty (Bug 2)

[ether-btc]

Bug 2: 75ms retry on empty results

Reported in community code audit

Problem

In retrieveWithRetry() (tools.ts), when the first retrieval returns 0 results, it always sleeps 75ms and retries — even when the store is completely empty. This 75ms is pure wasted latency on a cold/empty store.

Fix

Added count() method to MemoryStore using LanceDB's countRows() (O(1) metadata operation). retrieveWithRetry now accepts an optional countStore callback — if the store has 0 rows, the retry is skipped entirely.

// store.ts
async count(): Promise<number> {
  await this.ensureInitialized();
  return await this.table!.countRows();
}

// tools.ts — retrieveWithRetry
if (results.length === 0) {
  if (countStore) {
    const total = await countStore();
    if (total === 0) return results;  // skip retry
  }
  await sleep(75);
  results = await retriever.retrieve(params);
}

Testing

• TypeScript: 0 new errors on changed files
• Reviewed by Staff Engineer: APPROVED
• Backward compatible: countStore is optional, all 5 existing call sites still work

Files changed

• src/store.ts: +6 lines (count() method)
• src/tools.ts: +11/-5 lines (optional param + guard + 5 call sites)

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Audit reference: memory-lancedb-pro code audit — Bug 2, reported 2026-04-11