v0.19.0

Changed

• Reworked AppError storage to keep sources behind shared Arc handles and
  lazily capture optional Backtrace snapshots without allocating when
  RUST_BACKTRACE disables them.
• Updated the masterror::Error derive and ResultExt conversions to forward
  sources/backtraces automatically under the new storage layout.

Tests

• Added regression coverage for chained error sources and conditional
  backtrace capture driven by the RUST_BACKTRACE environment variable.