[Rust] Identity system in fd5 crate

[gerchowl]

Parent: #161

Goal

Add identity module to the fd5 Rust crate for loading ~/.fd5/identity.toml.

Spec

• Identity struct (serde): identity_type: String, id: String, name: String
• load_identity() -> Fd5Result<Identity> — read ~/.fd5/identity.toml, anonymous fallback
• save_identity(identity: &Identity) -> Fd5Result<()>
• Identity::anonymous() -> Identity — default anonymous identity
• Identity::to_author() -> Author — convert to Author struct for audit entries

TOML format

[identity]
type = "orcid"
id = "0000-0002-1825-0097"
name = "Lars Gerchow"

TDD

• Test load from valid TOML
• Test missing file → anonymous
• Test save + load round-trip
• Test identity type validation
• Test to_author conversion