Fix nix references in prose that no longer resolve

Nine of the 38 .nix tokens mentioned anywhere in *.rs did not name
anything that exists. Twelve mentions, five distinct targets:
hive-c0re.nix, hive-gateway.nix and hive-forge.nix are all directories
now; nix/modules/ is not a directory we have; hive-forge-tools.nix was
a bash script the binary replaced and is gone.

Where the reference is load-bearing it is corrected rather than
deleted, because the reference is the point: a comment saying a
constant must match a nix literal is only useful if you can open the
file it names. Where the module member was unambiguous the path now
names it exactly.

paths.rs's STATE_ROOT marker was the worst of them: it claimed the
value came from services.hyperhive.c0re.statePath, in hive-c0re.nix.
Neither exists. The option is not declared anywhere and the file is a
directory, so a "must match" contract pointed at two things that
cannot be opened. /var/lib/hyperhive is hardcoded on both sides, which
is what the comment now says.

hive-forge-tools.nix keeps no replacement: naming a file that was
deliberately deleted helps nobody, and "replaces a prior bash script"
is complete without it.

Measured before and after with the same command: 9 unresolved of 38
before, 4 of 35 after. The remaining four are an example path in a doc
comment, an upstream nixpkgs path, and two from one synthetic test
fixture.

hive-c0re/src/gateway_nginx.rs

@@ -97,7 +97,7 @@ fn render(names: &[String], frontend_dir: Option<&str>) -> String {
             // Split mode: try to serve files from the nix-store dist first;
             // fall through to the agent daemon for anything not found there.
             //
-            // `frontend` is a nix store path injected at build time by hive-c0re.nix
+            // `frontend` is a nix store path injected at build time by the hyperhive module
             // (`HIVE_AGENT_FRONTEND_DIR = "${cfg.frontend}/agent"`). Nix store paths
             // are of the form `/nix/store/<hash>-<name>` — only [a-z0-9/._-], never
             // shell metacharacters — so interpolating directly into the nginx config