mara on !2910: "transient guard as well - should be removable now?" — for
the queue path, yes.
`set_transient`'s own doc explained why the RAII guard existed: a
cancelled future must not leak an imperatively-set transient and pin the
dashboard on "rebuilding…" forever. That cannot happen to a derived set.
`running_transients()` is recomputed from the graph every loop, so a node
that stops running stops appearing — there is nothing to own and nothing
to leak.
So the scheduler no longer holds a guard per pill. It keeps the previous
derived value and publishes the transitions, which is the one thing a
derived read cannot express: the dashboard wants `TransientSet` /
`TransientCleared` edges, and the crash watcher wants the *moment* a pill
cleared, since its grace window is what stops an operator stop from
reading as a crash.
That also retires a hazard rather than restating it. The old code carried
a warning that stale guards had to be dropped before new ones were
created, because `TransientGuard::drop` clears by agent with no notion of
which label it was clearing — so a same-agent label change could clear
the pill it had just set. With no guards there is no ordering to get
wrong; clears are emitted before sets so a relabel reads as
clear-then-set rather than two overlapping pills.
`set_transient` / `clear_transient` become `pub(crate)`. The guard stays
for destroy and migration, which have no node behind them and where the
cancellation concern is real.
Checked with clippy (`--all-targets -D warnings`), `cargo test -p
hive-c0re -p hive-jobq` (322 + 41 passed) and `nix fmt`.
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