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/dashboard/mod.rs

@@ -115,7 +115,8 @@ pub async fn serve(
     webhook_secret: Option<String>,
 ) -> Result<()> {
     // API-only: the gateway static-serves the dashboard dist and proxies
-    // non-static requests here (see hive-gateway.nix). Unmatched paths 404.
+    // non-static requests here (see `nix/host-modules/hive-gateway/vhosts.nix`).
+    // Unmatched paths 404.
     // No static fallback needed here either. Every route (SSE streams
     // included — `utoipa::path` can document an SSE body as an opaque
     // `text/event-stream` string, it just can't model individual frame

hive-c0re/src/dashboard/state_snapshot.rs

@@ -76,7 +76,7 @@ pub(super) struct StateSnapshot {
     /// Whether the matrix GUI is reachable at `/matrix/`. Sourced from
     /// `HIVE_MATRIX_GUI_ENABLED` env var (set by the c0re NixOS module
     /// when `services.hyperhive.swarm.matrix.gui.enable` is on). The gateway
-    /// (hive-gateway.nix) does the actual `/matrix/` static serving;
+    /// (`nix/host-modules/hive-gateway/vhosts.nix`) does the actual `/matrix/` static serving;
     /// this flag is just an availability signal for iris's dashboard
     /// chrome so the `M4TR1X →` tab doesn't flash when the GUI is off.
     matrix_gui_enabled: bool,