fix: convert sovran-hosts-update to writeShellApplication with explicit runtimeInputs

- Replace environment.etc raw script with pkgs.writeShellApplication
- Declare runtimeInputs: pkgs.coreutils, pkgs.gawk, pkgs.gnugrep
- Use awk -v for safe marker variable passing (no shell interpolation)
- Point systemd ExecStart and activation script at lib.getExe hostsUpdateScript
- Keep /etc/sovran-hosts-update.sh as a source symlink for operator discoverability
- Remove environment.systemPackages reliance
- Emit warning (not silently swallow) on activation failure
- Add structural regression tests (19 new tests, all passing)

modules/core/local-domain-loopback.nix

@@ -32,22 +32,31 @@
 # regenerated by the system activation script.  The ``system.activationScripts``
 # hook below converts it to a writable file each time the system is activated
 # (i.e. after every ``nixos-rebuild switch``) and then injects the Sovran block.
-# The same script is also run by the ``sovran-hosts-update.service`` unit so
-# that the Hub can trigger it immediately after saving a domain without
-# requiring a full rebuild.
+# The same wrapped Nix-store executable is reused by both the activation hook
+# and the ``sovran-hosts-update.service`` unit, ensuring a deterministic runtime
+# PATH in every execution context.
 
-{
-  # ── Helper script (stored in the Nix store, never reads /var/lib at eval) ──
+let
+  # ── Wrapped Nix-store executable ──────────────────────────────────────────
+  # Built with pkgs.writeShellApplication so that all required runtime tools
+  # (awk, grep, coreutils) are declared explicitly and injected into PATH by
+  # Nix.  Both the systemd service and the activation hook reference this same
+  # store-path executable — there is no second raw script body.
+  hostsUpdateScript = pkgs.writeShellApplication {
+    name = "sovran-hosts-update";
 
-  environment.systemPackages = [ pkgs.coreutils ];
+    # Declare every external command the script calls.  These packages are
+    # added to the script's runtime PATH by writeShellApplication; nothing from
+    # the system PATH is relied upon.
+    runtimeInputs = [
+      pkgs.coreutils   # readlink, cp, mv, chmod, mktemp, rm, tr, head
+      pkgs.gawk        # awk
+      pkgs.gnugrep     # grep
+    ];
 
-  environment.etc."sovran-hosts-update.sh" = {
-    mode = "0755";
     text = ''
-      #!/bin/sh
       # Regenerate the Sovran-managed loopback block in /etc/hosts.
       # Safe to run multiple times — idempotent.
-      set -eu
 
       DOMAINS_DIR="/var/lib/domains"
       HOSTS_FILE="/etc/hosts"
@@ -66,13 +75,14 @@
 
       # ── Step 2: remove any existing Sovran block ──────────────────────────
       # Use a temp file so the operation is atomic.
+      # awk -v passes marker strings safely without shell interpolation.
       TMP=$(mktemp "$HOSTS_FILE.XXXXXX")
       trap 'rm -f "$TMP"' EXIT
-      awk "
-        /^$BEGIN_MARKER\$/ { skip=1; next }
-        /^$END_MARKER\$/   { skip=0; next }
+      awk -v begin="$BEGIN_MARKER" -v end="$END_MARKER" '
+        $0 == begin { skip=1; next }
+        $0 == end   { skip=0; next }
         !skip
-      " "$HOSTS_FILE" > "$TMP"
+      ' "$HOSTS_FILE" > "$TMP"
 
       # ── Step 3: collect valid configured service domains ──────────────────
       # NOTE: The hostname validation regex below must stay in sync with
@@ -110,6 +120,14 @@
     '';
   };
 
+in
+{
+  # ── /etc/sovran-hosts-update.sh — operator discoverability symlink ─────────
+  # Retain the familiar /etc path so administrators can inspect or manually
+  # invoke the helper.  The target is the wrapped Nix-store executable, so
+  # there is no second raw script body to keep in sync.
+  environment.etc."sovran-hosts-update.sh".source = lib.getExe hostsUpdateScript;
+
   # ── Systemd service ────────────────────────────────────────────────────────
 
   systemd.services.sovran-hosts-update = {
@@ -126,18 +144,24 @@
     serviceConfig = {
       Type = "oneshot";
       RemainAfterExit = true;
-      ExecStart = "/etc/sovran-hosts-update.sh";
+      # Point directly at the wrapped Nix-store executable, not the /etc path.
+      ExecStart = lib.getExe hostsUpdateScript;
     };
   };
 
   # ── Activation script (runs after every nixos-rebuild switch) ─────────────
   # This ensures the loopback block survives rebuilds that restore the /etc/hosts
   # symlink.  The "users" and "etc" scripts must complete first.
+  # The same wrapped Nix-store executable used by the systemd service is
+  # referenced here, guaranteeing identical runtime dependencies in both
+  # execution contexts.
 
   system.activationScripts.sovranDomainLoopback = {
     text = ''
-      if [ -x /etc/sovran-hosts-update.sh ] && [ -d /var/lib/domains ]; then
-        /etc/sovran-hosts-update.sh || true
+      if [ -d /var/lib/domains ]; then
+        if ! ${lib.getExe hostsUpdateScript}; then
+          echo "warning: sovran-hosts-update: failed to update /etc/hosts loopback entries" >&2
+        fi
       fi
     '';
     deps = [ "etc" "users" ];