fix: zero sensitive crypto key material after use

[LiranCohen]

Summary

Closes #78.

Zero ephemeral private keys, ECDH shared secrets, KEKs, CEKs, and intermediate HKDF derivation keys immediately after use to reduce the window where sensitive material remains in process memory.

Changes

• ecdh.go: defer clear() for ephemeral private key, shared secret, and KEK in ECDHESWrapKey; shared secret and KEK in ECDHESUnwrapKey
• jwe.go: defer clear(cek) in EncryptData, DecryptData, DecryptDataWithScheme
• hkdf.go: clear(currentKey) before reassignment and on error path in DeriveKeyBytes
• protocol.go: defer clear(protocolLevelKey) in InjectEncryptionDirectives, clear(childPrivateKey) after recursive call in injectEncryptionRecursive
• protocol.go: New Close() method on EncryptionKeyManager for graceful shutdown — zeros RootPrivateKey and all contextKeys

Design notes

• Go's GC can relocate heap objects, so zeroization is not a perfect guarantee, but it significantly reduces the attack surface by clearing keys as soon as they're no longer needed.
• Immutable Go strings (e.g., base64-encoded JWK D fields) cannot be zeroed; this is a known language limitation.
• EncryptionKeyManager.RootPrivateKey is needed for the process lifetime, so it gets a Close() method for graceful shutdown rather than immediate zeroization.
• Uses Go's built-in clear() (available since Go 1.21) for byte slices.

Verification

• go build ./... — zero errors
• go vet ./... — zero warnings
• go test ./... -count=1 -race — all tests pass, no data races