/**
 * @file Case-insensitive environment-variable key lookup —
 *   `findCaseInsensitiveEnvKey` walks `Object.keys(env)` with an O(1)
 *   length-prefilter before the (expensive) `toUpperCase` comparison. Used by
 *   `createEnvProxy` for Windows-style PATH/Path/path access.
 */
/**
 * Find a case-insensitive environment variable key match. Searches for an
 * environment variable key that matches the given uppercase name, using
 * optimized fast-path checks to minimize expensive toUpperCase() calls.
 *
 * **Use Cases:**
 *
 * - Finding PATH when env object has "Path" or "path"
 * - Cross-platform env var access where case may vary
 * - Custom case-insensitive env lookups
 *
 * **Performance:**
 *
 * - Fast path: Checks length first (O(1)) before toUpperCase (expensive)
 * - Only converts to uppercase when length matches
 * - Early exit on first match
 *
 * @example
 *   // Find PATH regardless of case
 *   const envObj = { Path: 'C:\\Windows', NODE_ENV: 'test' }
 *   const key = findCaseInsensitiveEnvKey(envObj, 'PATH')
 *   console.log(key) // 'Path'
 *   console.log(envObj[key]) // 'C:\\Windows'
 *
 * @example
 *   // Not found returns undefined
 *   const key = findCaseInsensitiveEnvKey({}, 'MISSING')
 *   console.log(key) // undefined
 *
 * @param env - Environment object or env-like record to search.
 * @param upperEnvVarName - Uppercase environment variable name to find (e.g.,
 *   'PATH')
 *
 * @returns The actual key from env that matches (e.g., 'Path'), or undefined
 */
export declare function findCaseInsensitiveEnvKey(env: Record<string, string | undefined>, upperEnvVarName: string): string | undefined;
