Make the resource lock unmisusable from outside the crate: the public
surface is now purely declarative (build a Graph with Dep::Resource edges,
configure capacities, run the Scheduler), and the scheduler owns every
acquire/release — a consumer never holds a guard, so it cannot hold the
lock wrong.
- `guard` module + `ResourceTable::try_acquire_all`/`release_all` +
`Graph::set_state` are now `pub(crate)`.
- Remove the dead borrowed-guard layer (`ResourceGuard::borrowed`,
`Acq::Borrowed`, `is_owning`): the scheduler tracks re-entrancy via its
own single borrow slot per (holder, resource) and never constructs a
borrowed guard, so re-entrancy lives in exactly one place. `Acq`
collapses into the owning `ResourceGuard` struct.
- `#[must_use]` on `Scheduler::settle` — ignoring its ids silently drops
runnable work.
- `SharedResources::with` (test-only table observability) is `#[cfg(test)]`.
- Drop the moot borrowed-guard tests; retained owning tests are black-box,
and the redundant `set_state` test helper is gone.
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