Keep library scans quiet by default

[Bortlesboat]

What changed

• make Scanner::new quiet by default for library consumers
• add an explicit with_open_port_output() opt-in used by the CLI
• replace the remaining UDP bind println! with debug logging
• cover both the quiet library default and the CLI opt-in

Why

Scanner::run() returns its discovered sockets, but it also printed each open socket directly to stdout. That made the library difficult to embed in applications that own their output. Keeping output disabled on a normal library-created scanner lets callers use the return value without unsolicited stdout, while the RustScan binary explicitly opts into the existing incremental display.

Fixes #706.

Validation

• cargo test scanner::tests — 11 passed
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

The complete cargo test invocation entered the repository's network/timing integration tests and exceeded a two-minute local command limit without reporting a failure; the full scanner unit suite completed successfully.