Bounded Concurrency Map (core/parallel.ts)

Runs promises with bounded concurrency to avoid overwhelming resources.

// Source: https://github.com/garrytan/gbrain | License: MIT | Copyright 2026 Garry Tan
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/**
 * Bounded-concurrency Promise.allSettled.
 *
 * Runs `fn(item)` for each item with at most `concurrency` in flight at a time.
 * Returns a `PromiseSettledResult` per input item, in input order, so callers
 * can distinguish fulfilled-with-result from rejected-with-error per item.
 *
 * Used by `gbrain sync --all` (v0.40 Federated Sync v2) to fan out per-source
 * syncs without overwhelming the embedding API or local disk.
 *
 * Why a semaphore + `Promise.allSettled` instead of a library:
 *   - Zero new deps. Bun's stdlib has every primitive we need.
 *   - `Promise.allSettled` semantics are what we want: one source failing
 *     mus

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