Tighten typed throws

[bradleymackey]

Summary

• Add explicit typed throws to production APIs whose thrown error type is unambiguous
• Promote narrow domain error types where public signatures need to expose them
• Keep broad protocol/framework-forwarding surfaces untyped

Verification

• CI green

Note: VaultExportTests has an .xctestplan but no runnable workspace scheme, and CI_iOS does not expose that plan directly.