SubtleCrypto deriveBits for PBKDF2 #345

crypto.subtle.deriveBits with PBKDF2 is needed by Fastly Compute apps that do key derivation for cookie/token schemes. A concrete example: the HUMAN Security (PerimeterX) _px3 cookie flow requires PBKDF2-HMAC-SHA256 (hundreds to thousands of iterations, 384 derived bits split into a 32-byte AES-256 key and 16-byte IV) followed by AES-256-CBC decryption. Without native PBKDF2, a JS implementation costs roughly ~45 ms per request versus ~1 ms natively, which is significant inside a Fastly Compute request handler.

Scope for the first increment:

• crypto.subtle.importKey("raw", keyData, "PBKDF2", extractable, usages) producing a secret CryptoKey usable for deriveBits/deriveKey (usages restricted to those two).
• crypto.subtle.deriveBits(algorithm, baseKey, length) wired through a CryptoAlgorithmDeriveBits base class, with the PBKDF2 operation implemented via OpenSSL PKCS5_PBKDF2_HMAC and the optional [EnforceRange] unsigned long length coercion semantics from the spec.
• SHA-1 / SHA-256 / SHA-384 / SHA-512 PRFs.

Follow-on work (not part of the first PR): deriveKey for PBKDF2, AES-CBC encrypt/decrypt to complete the _px3 flow, and the remaining HKDF/ECDH deriveBits algorithms.