Simplify checksum to use global crypto.subtle directly

CHANGELOG.md

@@ -4,8 +4,7 @@
 
 - Add `GtfsSqlJs.fromZipData(zipData, options?)` static method for loading from pre-loaded ZIP data (`ArrayBuffer` or `Uint8Array`)
 - Keep `GtfsSqlJs.fromZip(zipPath, options?)` strictly typed as `string` for paths/URLs
-- Remove `as unknown as Crypto` double cast in checksum module; call `webcrypto.subtle.digest()` directly
-- Remove `as BufferSource` cast in checksum module; pass `ArrayBuffer` directly to `sha256Digest()`
+- Simplify checksum module to use global `crypto.subtle` directly (available in both browsers and Node.js 18+, which is the minimum engine version); remove multi-branch environment detection, dynamic `import('crypto')` fallback, and empty `catch` block
 - Extract shared `isNodeEnvironment()` helper into `utils/env.ts`, replacing inline `typeof process` checks in zip-loader and gtfs-rt-loader
 - Narrow `loadGTFSZip()` parameter from `string | ArrayBuffer | Uint8Array` to `ArrayBuffer | Uint8Array` (string path was dead code)
 - Replace `unknown[]` with proper `ProtobufTimeRange[]` and `ProtobufEntitySelector[]` types in gtfs-rt-loader

src/cache/checksum.ts

@@ -1,31 +1,6 @@
 /**
- * Compute a SHA-256 digest using the Web Crypto API (browser or Node.js 18+).
- * Returns the raw hash as an ArrayBuffer.
- */
-async function sha256Digest(data: ArrayBuffer): Promise<ArrayBuffer> {
-  // Browser environment
-  if (typeof crypto !== 'undefined' && crypto.subtle) {
-    return crypto.subtle.digest('SHA-256', data);
-  }
-
-  // Node.js environment
-  if (typeof globalThis !== 'undefined') {
-    try {
-      const { webcrypto } = await import('crypto');
-      if (webcrypto?.subtle) {
-        return webcrypto.subtle.digest('SHA-256', data);
-      }
-    } catch {
-      // Fallback for older Node.js versions
-    }
-  }
-
-  throw new Error('Web Crypto API not available in this environment');
-}
-
-/**
- * Compute SHA-256 checksum of data
- * Uses Web Crypto API (available in both browser and Node.js 18+)
+ * Compute SHA-256 checksum of data.
+ * Uses the global Web Crypto API (available in browsers and Node.js 18+).
  */
 export async function computeChecksum(data: ArrayBuffer | Uint8Array): Promise<string> {
   let buffer: ArrayBuffer;
@@ -37,7 +12,7 @@ export async function computeChecksum(data: ArrayBuffer | Uint8Array): Promise<s
     new Uint8Array(copy).set(data);
     buffer = copy;
   }
-  const hashBuffer = await sha256Digest(buffer);
+  const hashBuffer = await crypto.subtle.digest('SHA-256', buffer);
   const hashArray = Array.from(new Uint8Array(hashBuffer));
   const hashHex = hashArray.map(b => b.toString(16).padStart(2, '0')).join('');
   return hashHex;