apple: fall back to non-userPresence storage when the keychain rejects the ACL

[jgowdy-godaddy]

Summary

• Detect errSecMissingEntitlement (-34018) and errSecParam (-50) responses from SecItemAdd on the userPresence path and retry without kSecAttrAccessControl
• Emit a one-line stderr warning so the config-level downgrade is visible, not silent
• Signed-build / Data-Protection-keychain path is unchanged — fallback only runs after the primary add has already failed

Motivation

Since #77 made userPresence the default in sshenc (#50), sshenc keygen fails on every unsigned macOS build (cargo, Homebrew) because neither keychain can honor the .userPresence SecAccessControl flag:

Keychain	Problem
Data Protection	errSecMissingEntitlement (-34018) — caller isn't code-signed with the DP-keychain entitlement
Legacy	errSecParam (-50) — doesn't support .userPresence on SecAccessControl at all

The existing top-of-section comment explicitly calls the entitlement problem out ("on unsigned builds … the modern Data Protection keychain returns errSecMissingEntitlement"), but the userPresence code path skipped over it.

Impact

Before this change, on my unsigned dev build:

$ sshenc keygen --label new
Error: Secure Enclave operation failed: generate: key operation failed:
       keychain_store: Swift bridge returned error code 9

After:

$ sshenc keygen --label new
enclaveapp: wrapping-key userPresence ACL rejected (OSStatus=-34018);
 falling back to non-userPresence storage — userPresence gate won't fire for this key
Generated Secure Enclave key: new
  …

The user keeps Secure Enclave's AccessPolicy-driven prompts at signing time (independent of the wrapping-key ACL). They lose the additional userPresence gate on wrapping-key decrypt until they sign the binary. The warning is loud enough to not be silent security-config drift.

Test plan

• cargo test --workspace — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --all -- --check — clean
• sshenc e2e suite on unsigned dev build: 36/36 pass in SE mode (same as software mode)
• Downstream sshenc e2e on signed macOS-latest GH runner (CI of the sshenc-side follow-up PR will prove this path)