[PR #30] tracing subscriber writes to stdout instead of stderr

[obchain]

PR: #30 (feat/05-clap-cli)
File: crates/charon-cli/src/main.rs
Lines: ~42-46

Problem: tracing_subscriber::fmt() defaults to stdout. listen subcommand intended to eventually emit structured JSON to stdout for downstream consumers. Mixing log output with data output on same fd breaks that contract and silently corrupts any JSON consumer.

Fix: Add .with_writer(std::io::stderr) to fmt() builder:

tracing_subscriber::fmt()
    .with_env_filter(...)
    .with_writer(std::io::stderr)
    .init();

[obchain]

Fixed on feat/05-clap-cli in a235488.