/**
 * Pull the `Idempotency-Key` header from a Node-style headers bag,
 * case-insensitive. Returns `undefined` when any of the following
 * carries no usable key:
 *
 * - the header is missing
 * - its value is an array (ambiguous — can't pick one without a
 *   policy the receiver hasn't declared)
 * - its value is the empty string (carries no idempotency
 *   information; structurally equivalent to "no header at all")
 *
 * Pair with `IdempotencyStore.claim` from
 * `@rotorsoft/act-ops/idempotency`: extract the key from the inbound
 * request, claim it on the store, return a `deduped` marker when the
 * claim fails. The framework-agnostic middleware that wires these
 * together lands in #744.
 *
 * Validation beyond "is there a usable key?" (length bounds, format
 * checks, normalization) is intentionally out of scope. Receivers
 * picking a policy can layer it on top — or, when #744 ships, opt
 * into the middleware's opinionated defaults.
 */
export declare function extractIdempotencyKey(headers: Record<string, string | string[] | undefined>): string | undefined;
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