`Graph::insert` was public and validating, and the builder's insert loop
called it with `?`. That made job-level atomicity an accident: the loop
mutates as it goes, so a rejection at node `i` left `0..i` already in the
graph — the error fired loudly *after* the corruption, not instead of it.
It held only because `check_job_shape` happens to be exhaustive, with
nothing in the types saying so.
Make the guarantee structural instead. `Graph::insert` becomes
`pub(crate)`; the builder drains into a new infallible
`insert_unchecked` (via `Scheduler::append_unchecked`), so
`insert_with`'s sink returns a bare `NodeId` and a half-built job is no
longer expressible. Re-checking at the sink cannot add safety anyway — it
can only report after the mutation it was meant to prevent.
Drop `BuildError::Graph`: nothing in the builder path can produce a
`GraphError` any more. Clippy could not see this (an unreachable variant
of a `pub` enum is still constructible from outside the crate).
`Scheduler::append` stays public and validating — hive-c0re inserts a
DAG's container node through it. Folding that away means removing the
container/DagView indirection, which is out of scope here.
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