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/resources.rs

@@ -7,7 +7,7 @@
 //! available-once". Typical names: `build-slot` (capacity = number of build
 //! slots), `agent/<name>` (capacity 1 — the per-agent lifecycle lock).
 //!
-//! The one operation that matters is [`ResourceTable::try_acquire_all`]: it
+//! The one operation that matters is `ResourceTable::try_acquire_all`: it
 //! takes *all* of a node's resource requests and either grants every one or
 //! grants none, touching nothing on failure. Because a node acquires all its
 //! resources atomically at start (never holds one while waiting for another),
@@ -27,8 +27,8 @@ use std::hash::Hash;
 ///
 /// Configure known capacities with [`ResourceTable::set_capacity`]; any name
 /// left unconfigured has the default capacity (1). Acquire and release move
-/// units atomically via [`ResourceTable::try_acquire_all`] /
-/// [`ResourceTable::release_all`].
+/// units atomically via `ResourceTable::try_acquire_all` /
+/// `ResourceTable::release_all`.
 #[derive(Debug, Clone)]
 pub struct ResourceTable<R> {
     /// Configured capacities, keyed by resource. Missing ⇒ `default_capacity`.