v0.16.0

Changed

• Switched the internal AppError source storage to Arc<dyn Error> and added a
  shared with_source_arc helper so conversions can reuse existing Arc
  handles without extra allocations.
• Replaced the backtrace slot with an Option<Backtrace> managed through an
  environment-aware lazy capture that respects RUST_BACKTRACE and avoids
  snapshot allocation when disabled.
• Updated the masterror::Error derive and ResultExt conversions to forward
  sources using the new shared storage while preserving error chains.

Tests

• Added regression coverage for the std::error::Error chain, Arc source
  preservation in the derives, and conditional backtrace capture driven by the
  RUST_BACKTRACE environment variable.