v0.0.8

Add Running() method to runnable Group (#7)

Add `Running()` method to `Group`.

## Summary
- `Group.Running()` reports whether `Run` is in progress (including during shutdown); it is not a health signal — use `Alive()` for liveness/readiness
- Overlapping `Run` calls return `ErrAlreadyRunning` via `CompareAndSwap`
- Empty groups never report `Running() == true`
- Covered by `TestRunning`, `TestRunningEmptyGroup`, and `TestRunAlreadyRunning`

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Co-authored-by: Cursor <cursoragent@cursor.com>