feat: logging improvements

[vmarcella]

• Replace unsound static mut global with OnceLock<Arc>; Logger::global() now thread-safe
• Unify handler API: pub trait Handler: Send + Sync { fn log(&self, record: &Record) }
• Introduce Record (timestamp, level, target, message, module/file/line) passed to handlers
• Make Logger concurrent: AtomicU8 for level; RwLock<Vec<Box>> for handlers; methods
  take &self
• Remove implicit process exit from fatal; it now only logs (explicit exit left to the app)
• Add low-overhead logging helpers: enabled(level) and log_args(level, module, file, line, args)
• Rewrite macros (trace!/debug!/info!/warn!/error!/fatal!) to use $crate, early level guard, and
  format_args! with metadata
• Improve ConsoleHandler: color only when attached to a TTY; write WARN/ERROR/FATAL to stderr,
  others to stdout
• Keep FileHandler behavior but make it thread-safe (buffer protected by Mutex; flush every 10
  messages)
• Add Logger::builder() API (name, level, .with_handler(...)) for ergonomic configuration
• Add environment config module:
  • env::parse_level(&str) -> Option
  • env::apply_env_level(&Logger, Option<&str>) (defaults to LAMBDA_LOG)
  • env::init_global_from_env() to create a global console logger honoring LAMBDA_LOG
• Add JsonHandler (newline-delimited JSON) and RotatingFileHandler (size-based rotation with
  backups)
• Update README with new usage (builder, env config, console behavior) and clarify import options
• Add comprehensive logging guide: docs/logging-guide.md (overview, config, handlers, examples,
  changelog)
• Add improvement spec doc: docs/logging-improvements-spec.md with metadata and phased plan
• Add runnable examples:
  • 01_global_macros.rs (macros)
  • 02_custom_logger.rs (custom instance)
  • 03_global_init.rs (init custom global)
  • 04_builder_env.rs (builder + env level)
  • 05_json_handler.rs (JSON output)
  • 06_rotating_file.rs (size-based rotation)
• Add tests covering:
  • Global singleton identity; level filtering; handler order; multi-threaded logging
  • fatal behavior (no exit); macro early-guard (no formatting when disabled)
  • Builder config; env parse/apply; JSON write; rotation behavior
• Verified locally:
  • cargo test (workspace): all pass
  • cargo clippy -p lambda-rs-logging --all-targets -- -D warnings: clean
  • cargo build -p lambda-rs-logging --examples: builds all examples
• Breaking changes:
  • Handler implementations must migrate to fn log(&self, &Record)
  • fatal! no longer exits (apps should exit explicitly if desired)