v0.14.0

Added

• Introduced optional tracing, metrics and backtrace features. When
  enabled they emit structured tracing events, increment the
  error_total{code,category} counter and capture lazy [Backtrace] snapshots
  from a new AppError::emit_telemetry hook.

Changed

• Reworked the AppError core to emit telemetry exactly once, track dirty
  mutations and expose a crate-private new_raw constructor for contexts that
  enrich errors before flushing instrumentation.
• Updated Axum and Actix integrations to rely on the telemetry hook instead of
  manually logging errors while preserving backward-compatible APIs.

Tests

• Added tracing dispatcher coverage to assert a single telemetry event with MDC
  propagated trace_id values.
• Installed a deterministic metrics recorder in unit tests to confirm
  error_total increments once per error.