451

[RAprogramm]

Closes #451

Implements real AppError::downcast and downcast_mut (both were stubs that unconditionally failed).

Design

• Source storage becomes StoredSource: Owned(Box<dyn Error + Send + Sync>) / Shared(Arc<...>). Needed because an Arc<dyn Error> can never yield exclusive ownership in safe Rust (try_unwrap requires Sized), and the crate forbids unsafe.
• downcast::<E>(self) -> Result<Box<E>, Self>: succeeds for an owned immediate source of type E; on failure the error is returned fully intact (box reinserted). Shared sources always return Err — documented with rationale (anyhow does this with unsafe; we don't).
• downcast_mut::<E>(): owned → mutable box downcast; shared → Arc::get_mut (unique reference only, mirroring anyhow 1.0.103's soundness semantics). No unsafe anywhere.
• is/downcast_ref semantics unchanged (immediate source); with_source now stores owned, with_source_arc shared — ownership caveats documented; ResultExt::context sources become downcastable as a bonus.

Tests

15 new tests: owned success, wrong-type Err with error intact, shared-Arc Err with strong_count preserved, unique-Arc downcast_mut success, shared-Arc None, mutation visible via downcast_ref, interplay of with_source / with_source_arc / with_context. Stub-era tests replaced. Full suite, doctests (173), no_std build, clippy zero warnings, nightly fmt — all green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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