fix(perf): precompile health regexes, share HTTP client, cut hot-path clones

[iamquang95]

Fix #645

• app/health — a regex was compiled per metric series per check tick; now compiled once into a static.
• cli/test — a new reqwest::Client per probe meant each RTT measurement included its own TCP/TLS handshake; now one shared client.
• core — DutyType::Display round-tripped through serde_json on every call; now a direct as_str().
• parsigex — do_recv deep-cloned a ParSignedData (a whole beacon block for proposals) per entry per message; now borrowed.
• p2p — swapped an allocating .is_empty() check for the existing has_connection.

Behaviour change: request_rtt and relay_ping_test had no request timeout; they now use the 10s one beacon.rs/mev.rs already applied. Sharing a client also means concurrent probes to one host multiplex over a single HTTP/2 connection — only visible in beacon_ping_load_test.