Part 1 (from C1 review): add `OrchestratorError::AllFailed(String)` variant
to replace the misused `Internal` in `aggregate_concurrent_verdict`; update
the doc-comment on that function; update `cleanup_tests_c1.rs` assertions.
Part 2 (Task C2): `Orchestrator::run()` now computes `screen_groups` +
`effective_concurrency` after planning and branches:
- `effective > 1` → N-root scaffold (`build_scaffold_concurrent_mobile`) +
`run_concurrent` + `aggregate_concurrent_verdict` + cleanup; on all-fail
calls `cleanup_concurrent_roots` and returns `AllFailed`.
- `<= 1` → the existing sequential path, completely unchanged.
Inline tests extracted to sibling files (`run_tests.rs`, `run_tests_c2.rs`)
to keep `run.rs` under the 800-line cap. 226 tests pass.
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