Task B2 of S3b-2: implements `run_concurrent` — the concurrent executor
that drives screen-group workers via a tokio Semaphore (RAII permits,
FIFO-fair, cap = effective_concurrency), collects per-worker buffered
EditorCommands, replays them into the real DocSink in subtask-plan-index
order, and fan-ins Progress events via mpsc channel.
Also splits concurrent.rs test block into concurrent_tests.rs (STEP 0)
to keep both files under the 800-line ceiling, wired via
`#[path = "concurrent_tests.rs"] mod tests` (same pattern as plan_repair).
Updated run_screen_group_worker signature: now takes Arc<Semaphore> +
mpsc::UnboundedSender<Progress> instead of &mut dyn FnMut(Progress).
Added CountingLlm to test_support for semaphore-cap verification.
Added tokio (sync + rt + macros) to op-orchestrator Cargo.toml.
208 tests pass; clippy -D warnings clean; fmt clean.
concurrent.rs: 432 lines; concurrent_tests.rs: 727 lines.
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