Function init 

pub fn init(filter: Option<&str>, default_filter: &str)

Initialize a compact, human-friendly tracing subscriber.

Priority for the filter is:

1. Explicit filter argument (if provided)
2. RUST_LOG / default env filter
3. default_filter

Also enables the log-to-tracing bridge so log crate messages (e.g., from yamux) are filtered by the same rules.