Production-grade structured logging (v0.9.0)

[guyte149]

Replace the global-max-level subscriber with tracing-subscriber + EnvFilter, add --quiet and --log-format compact|json, and unify every log call site around a stable span schema that mirrors the rusnel ctl ID model (client_id / tunnel_id / conn_id).

Why

• The old subscriber was a single global level knob with no per-module control and no way to gate noisy upstream crates separately, so --debug drowned rusnel's own messages in quinn / rustls trace output.
• Field names drifted between sites (id vs client_id, local TCP counter id vs admin conn_id, Forward vs "forward"), which made greppable correlation with the admin API unreliable.
• Connection lifecycles only logged "opened"; closing was silent unless there was an error, so basic "who used the most bytes" questions required either standing up an aggregator or shelling into the admin socket.

What changes

• init_logging in src/main.rs: EnvFilter honoring RUST_LOG, synthesized defaults that keep quinn/rustls/h3/hyper at WARN while rusnel modules respect -v / --debug / -q. Compact format with ISO-8601 UTC millisecond timestamps, target hidden, ANSI auto-detected from stderr TTY-ness. --log-format json emits one event per line for log aggregators.
• Stable spans: client{client_id,peer} (server) / session{peer} (client) → tunnel{tunnel_id,dir,spec} → conn{conn_id,tunnel_id, peer,proto}. Server-side conn span now uses the real conn_id from ServerState::register_conn; client-side falls back to a local counter when admin tracking is off.
• Every conn emits structured conn opened / conn closed events; the close line carries bytes_in, bytes_out (snapshotted from the ConnGuard before drop), and dur_ms.
• Subcommand subscribers (ctl, cert) collapsed into a shared init_simple_logger so they pick up RUST_LOG overrides too.
• Cargo.toml: enabled the env-filter/fmt/ansi/json/time features on tracing-subscriber and added time for the timestamp format description.
• Messages rewritten throughout server/, client/, common/{tcp,udp, socks,proxy,tunnel,quic}, server/admin, lib: lowercase, terse, present-tense, key-value structured fields instead of inline format args.
• README: marked the RUST_LOG TODO done and added a Logging section with the schema, flags, and a real example trace.