A node no longer hands back a recipe for the scheduler to replay later. It
declares straight onto a builder it was given, and that builder is inserted
as part of completing the node.
Deleted: `pub type Declare`, `struct NodeOutput` (+ its hand-written `Debug`),
`JobQueue::append_subgraph`. Nothing added to `Dag` / `DagView`.
jobq gains `Scheduler::new_job()` (the only way to obtain a `JobBuilder`) and
`complete_growing(id, outcome, grown)`, which inserts under `id` and *then*
completes it, so a DAG cannot roll terminal while grown work is still pending.
`complete()` and `complete_growing()` share a private `finish()` rather than
one redirecting through the other. The DAG-gone guard lives beside the graph
now, where it cannot be skipped, instead of being a caller-side lookup.
The growth executors return data (`run_meta_lock -> (Vec<String>, RebuildOpts)`,
`run_reconcile -> Option<NodeKind>`) rather than taking the builder: a `&Job`
parameter is live for the whole function body, and `&RefCell<T>` is never
`Send`, so an async fn taking one cannot be spawned. `run_node` threads the
builder by value and hands it back.
A node can now declare work and then fail, which was previously inexpressible.
`grown` is dropped in that case — failure cancel-cascades downstream, so
inserting it would only add nodes to immediately cancel — and the log line
carries `grown_nodes` so the drop is visible.
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