* feat(signals): add WritableResultExt trait
Add WritableResultExt mirroring the existing ReadableResultExt,
providing mutable access to signal values holding Result<T, E>.
Methods:
- as_mut(): returns Result<WritableRef<T>, WritableRef<E>>
- unwrap_mut(): returns WritableRef<T>, panics if Err
Closes#5293
* refactor(signals): use descriptive expect messages in WritableResultExt::as_mut
Replace bare .unwrap() calls with .expect() containing a descriptive
message about the failure mode. This gives better diagnostics if the
Result variant changes between the read and write lock acquisitions.
* switch to a single write to avoid race conditions with sync signals
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Co-authored-by: Evan Almloff <evanalmloff@gmail.com>
