Implement CR8Client per create-protocol/cr8/specs/arka-cr8-client.md

[abhicris]

Implement the `CR8Client` trait and the `CR8ClientBuilder` constructor per the spec merged in create-protocol/cr8#18.

Scope

• `arka::cr8` module exposing the trait + builder verbatim per spec §2.
• `switchboard::Wallet`-only signing path (no private-key constructor).
• Full `CR8Error` enum per spec §3.
• `CR8ClientRecovery` sibling trait (spec §2.5).
• Per the spec §5 versioning rules, pin `cr8-spec` (or inline) at minor-range.

Acceptance

• `cargo test` covers register / deposit / withdraw / claim / complete / balance / watch happy paths against a local Anvil instance.
• One integration test runs the spec §4 end-to-end example against Arbitrum Sepolia and exits clean.
• README links back to the merged spec by commit-pinned URL.

Spec source

• `create-protocol/cr8/specs/arka-cr8-client.md`
• Sibling specs: `switchboard-integration`, `arbitrum-cli-cr8-subcommand`