hive-c0re: stop naming nix implementation files in env-var prose

Six diagnostics and eight comments explained where an environment
variable comes from by naming the module file that sets it. The reader
of a panic is an operator, who does not edit that file; the reader of
the comment is already in the code. Neither can act on a filename, and
a filename in prose is checked by nothing.

Each one now names what the reader can act on: the option where there
is one (services.hyperhive.network.bridgeName, .bridgeIp,
.bridgePrefixLength, c0re.assets, c0re.orgAvatarPng), otherwise the
unit the process is expected to run under. Two of the six diagnostics
had a real option available; the other four have none, so they name
the unit rather than an invented path.

Four of them pointed at hive-c0re.nix, which is a directory. Two of
those spelled it nix/modules/hive-c0re.nix — a directory that does not
exist either, and one of the two was wrapped across a line break, so
no grep for the whole path could have found it. That is the argument
for the change: prose naming a file rots silently because nothing
compiles it.

Scope here is the four files around those diagnostics. The same
pattern still matches elsewhere in the tree; that sweep follows.

hive-c0re/src/lifecycle/mod.rs

@@ -126,8 +126,8 @@ pub fn bridge_gateway_ip(subnet_cidr: &str) -> Option<String> {
 }
 
 /// Build the network-isolation settings every container is configured
-/// with, from the variables `hive-network.nix` sets on the `hive-c0re`
-/// unit.
+/// with, from the variables the hyperhive NixOS module derives from
+/// `services.hyperhive.network.*` onto the `hive-c0re` unit.
 ///
 /// Isolation is the only supported mode: the on/off toggle is gone, so
 /// there is no non-isolated branch to fall back to and a missing or
@@ -143,12 +143,14 @@ pub fn network_isolation_from_vars(
     subnet: Option<&str>,
 ) -> Result<hive_priv_sock::NetworkIsolation> {
     let bridge = bridge.filter(|s| !s.is_empty()).context(
-        "HIVE_NETWORK_BRIDGE is unset or empty — hive-network.nix sets it on the \
-         hive-c0re unit, so this means the daemon is running outside its unit or \
-         with a broken module evaluation",
+        "HIVE_NETWORK_BRIDGE is unset or empty — the hyperhive NixOS module sets it \
+         from services.hyperhive.network.bridgeName on the hive-c0re unit, so this \
+         means the daemon is running outside its unit or with a broken module \
+         evaluation",
     )?;
     let subnet = subnet.filter(|s| !s.is_empty()).context(
-        "HIVE_NETWORK_SUBNET is unset or empty — hive-network.nix sets it on the \
+        "HIVE_NETWORK_SUBNET is unset or empty — the hyperhive NixOS module sets it \
+         from services.hyperhive.network.bridgeIp and .bridgePrefixLength on the \
          hive-c0re unit, so this means the daemon is running outside its unit or \
          with a broken module evaluation",
     )?;