feat(#3245): gate rustdoc in nix flake check, and clear the workspace

Nothing in the gate read doc-comments: clippy doesn't check intra-doc
links, cargo test doesn't, and no check built docs. So a [`Foo`] pointing
at a renamed, moved or deleted item rendered as plain text and had no
discoverer but a human happening to read the comment.

That matters here more than in most repos, because the convention is to
put a thing's authoritative description in one doc-comment and point at
it from everywhere else -- the design leans on the pointers being real,
and a dangling link is worse than no link since it names something and
sends the reader looking.

Adds `docs-rustdoc` to nix/checks.nix: craneLib.cargoDoc over
--workspace --no-deps --document-private-items, denying six rustdoc
lints. Listed explicitly rather than -D warnings so a new lint appearing
upstream cannot red the build on a class nobody has triaged.

--document-private-items is load-bearing rather than thoroughness for
its own sake: most of this workspace's doc-comments live on private
items and //! module headers, so without it rustdoc checks a small
fraction of the links and the gate sits green while the rot continues.

Then fixes every error it reports, 40 to 0 across nine crates. The
classes differ and so do the fixes:

- public item, wrong scope -> qualify. Node and Node::parent are both
  public; the link failed only because scheduler.rs does not import
  Node. Six sites become [`crate::Node::parent`].
- private item -> downgrade to backticks. Nothing was made public to
  satisfy a lint; changing API surface to appease a doc check would be
  the tail wagging the dog.
- genuinely dead -> [`JobBuilder::insert_into`] names a method that does
  not exist. Insertion is Scheduler::insert_job.
- prose that looks like markup -> argv[0] parsed as a link, and
  <args>/<hex>/<name> parsed as HTML tags.

Note for future fixes: pub(crate) resolves in an intra-doc link, a plain
private fn in a binary crate does not (wait_for_nodes resolved,
connect_hint did not, same crate, same shape).

The check does not ride the clippy/test artifact cache. It takes
cargoArtifacts, but rustdoc needs its own flavour of dependency
metadata, which cargo build does not produce, so a --no-deps docs build
still compiles dependencies it never documents. Measured at 6m47s cold;
that reasoning is recorded in the check's own comment so the next reader
does not re-derive it.

Verified by running the check's exact command against the pre-cleanup
tree first: 40 errors, build failed. A gate that cannot fail is not
evidence, and building it before the cleanup makes that proof free.

hive-jobq/src/builder.rs

@@ -12,7 +12,7 @@
 //! inserted from outside this crate — `new()` and `insert_with` are both
 //! `pub(crate)`, and there is deliberately no `Default` impl, since a trait impl
 //! on a `pub` type is public regardless. There is no intermediate
-//! node-description type to keep in sync with [`crate::Graph::insert`]'s signature —
+//! node-description type to keep in sync with `Graph::insert`'s signature —
 //! so a job has no representation that can be passed around instead of being
 //! inserted.
 //!
@@ -115,7 +115,7 @@ pub enum BuildError {
 /// Reject a job whose own declarations don't hold up — **before anything is
 /// inserted**, so no failure can leave a partial job behind.
 ///
-/// This covers *every* rejection [`crate::Graph::insert`] can raise for a
+/// This covers *every* rejection `Graph::insert` can raise for a
 /// builder-produced node, which is what makes the insert loop below infallible
 /// in practice:
 ///
@@ -387,7 +387,7 @@ impl<N, R> From<&NodeRef<'_, N, R>> for NodeGuid {
 impl<N, R> NodeRef<'_, N, R> {
     /// This node's handle, for callers that want to hold the identity without
     /// the builder borrow (e.g. to look the id up after
-    /// [`JobBuilder::insert_into`]).
+    /// [`crate::scheduler::Scheduler::insert_job`]).
     #[must_use]
     pub fn guid(self) -> NodeGuid {
         self.guid