feat(log): emit program identity banner + module target on every line (closes #310)

[0xghost42]

Why

Issue #310: the existing log file only carries timestamp + level + message, so a user looking at a long-running rustnet log has no direct way to confirm which binary, which version, and which pid produced the entries.

What

Two small changes in setup_logging:

1. Module target on every line. Switch from LogConfig::default() to a ConfigBuilder with set_target_level(LevelFilter::Error), which tells simplelog to include the originating target (module path) on every log line.

   Before:

   2026-05-25 18:00:00 [INFO] Refreshing connections
   

   After:

   2026-05-25 18:00:00 [INFO] (rustnet_monitor::network::poll) Refreshing connections
   

2. Startup banner with program identity. Emit one identifying line immediately after the writer is wired up:

   2026-05-25 18:00:00 [INFO] (rustnet_monitor::main) rustnet-monitor v1.3.0 starting (pid 12345)
   

   pkg_name and pkg_version come from env!("CARGO_PKG_NAME") / env!("CARGO_PKG_VERSION") so they stay correct when the binary is renamed or symlinked (i.e. not derived from argv[0]). The pid uses std::process::id().

Verification

$ cargo build              # clean
$ cargo clippy --all-targets -- -D warnings   # clean
$ cargo fmt --check        # clean
$ cargo test --lib         # 365 passed

No behavior change outside the log file format. Open to feedback if you'd prefer the banner phrased differently (e.g. include the BPF filter / interface in the startup line, or include the start timestamp explicitly even though simplelog already stamps each line).

Closes #310

[domcyrus]

I'm curious with this and the one you've added further down we have two starting RustNet log lines, should they be combined.

[domcyrus]

@0xghost42 Thanks a lot for your PR, Ive added one minor comment which I think we should tackle prior merging this.