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)

app/tests/test_local_domain_loopback_nix.py

@@ -0,0 +1,130 @@
+"""Structural regression tests for modules/core/local-domain-loopback.nix.
+
+Verifies that the sovran-hosts-update helper:
+  - is built as a pkgs.writeShellApplication with explicit runtimeInputs
+    (gawk, gnugrep, coreutils);
+  - uses ``lib.getExe hostsUpdateScript`` for both the systemd ExecStart and
+    the activation script so that both contexts share the same Nix-store
+    executable;
+  - does NOT point ExecStart at the raw /etc path;
+  - includes element-calling in the supported domain list;
+  - uses ``awk -v`` for safe marker-variable passing rather than interpolating
+    marker text directly into the awk program;
+  - retains the domain validation regex (idempotency / injection prevention);
+  - exposes /etc/sovran-hosts-update.sh as a symlink via ``source =`` (not a
+    second raw ``text =`` body);
+  - does NOT rely on environment.systemPackages for the helper's dependencies.
+"""
+
+import unittest
+from pathlib import Path
+
+NIX_FILE = (
+    Path(__file__).resolve().parents[2]
+    / "modules"
+    / "core"
+    / "local-domain-loopback.nix"
+)
+
+
+class LocalDomainLoopbackNixStructureTests(unittest.TestCase):
+    def setUp(self):
+        self.source = NIX_FILE.read_text()
+
+    # ── writeShellApplication and explicit runtimeInputs ────────────────────
+
+    def test_uses_write_shell_application(self):
+        self.assertIn("pkgs.writeShellApplication", self.source)
+
+    def test_runtime_inputs_includes_coreutils(self):
+        self.assertIn("pkgs.coreutils", self.source)
+
+    def test_runtime_inputs_includes_gawk(self):
+        self.assertIn("pkgs.gawk", self.source)
+
+    def test_runtime_inputs_includes_gnugrep(self):
+        self.assertIn("pkgs.gnugrep", self.source)
+
+    def test_runtime_inputs_block_present(self):
+        self.assertIn("runtimeInputs", self.source)
+
+    # ── Both execution paths use lib.getExe ─────────────────────────────────
+
+    def test_exec_start_uses_lib_get_exe(self):
+        """systemd ExecStart must reference the Nix-store executable."""
+        self.assertIn("ExecStart = lib.getExe hostsUpdateScript", self.source)
+
+    def test_activation_script_uses_lib_get_exe(self):
+        """Activation text must call the same Nix-store executable."""
+        self.assertIn("${lib.getExe hostsUpdateScript}", self.source)
+
+    def test_exec_start_does_not_point_to_etc_path(self):
+        """ExecStart must NOT use the raw /etc path (which lacks a deterministic PATH)."""
+        self.assertNotIn('ExecStart = "/etc/sovran-hosts-update.sh"', self.source)
+
+    # ── /etc symlink uses source =, not a second text = body ────────────────
+
+    def test_etc_entry_uses_source_not_text(self):
+        """The /etc/sovran-hosts-update.sh entry must be a symlink (source =),
+        not a second raw script body (text =)."""
+        self.assertIn(
+            'environment.etc."sovran-hosts-update.sh".source', self.source
+        )
+
+    def test_etc_source_points_to_get_exe(self):
+        self.assertIn(
+            'environment.etc."sovran-hosts-update.sh".source = lib.getExe hostsUpdateScript',
+            self.source,
+        )
+
+    # ── element-calling domain is supported ─────────────────────────────────
+
+    def test_element_calling_domain_key_present(self):
+        self.assertIn("element-calling", self.source)
+
+    # ── Robust awk -v variable passing ──────────────────────────────────────
+
+    def test_awk_uses_dash_v_for_begin_marker(self):
+        """awk must receive the begin marker via -v, not by shell interpolation."""
+        self.assertIn('awk -v begin=', self.source)
+
+    def test_awk_uses_dash_v_for_end_marker(self):
+        self.assertIn('-v end=', self.source)
+
+    def test_awk_does_not_interpolate_marker_into_program(self):
+        """The old pattern interpolated $BEGIN_MARKER directly into the awk source."""
+        self.assertNotIn('/$BEGIN_MARKER', self.source)
+        self.assertNotIn('/$END_MARKER', self.source)
+
+    # ── Domain validation ────────────────────────────────────────────────────
+
+    def test_domain_validation_regex_present(self):
+        """The hostname validation regex must still be present for injection prevention."""
+        self.assertIn("grep -qE", self.source)
+        self.assertIn("[a-zA-Z0-9]", self.source)
+
+    def test_invalid_domain_warning_present(self):
+        self.assertIn("skipping invalid domain value", self.source)
+
+    # ── No environment.systemPackages reliance ───────────────────────────────
+
+    def test_no_environment_system_packages_for_helper(self):
+        """The helper's tools are declared via runtimeInputs; the module must
+        not add them to environment.systemPackages."""
+        self.assertNotIn("environment.systemPackages", self.source)
+
+    # ── Idempotency: existing Sovran block is removed before rewriting ───────
+
+    def test_existing_block_removal_logic_present(self):
+        """awk strip of the managed block must be present for idempotency."""
+        self.assertIn("skip=1", self.source)
+        self.assertIn("skip=0", self.source)
+
+    # ── Activation script warns on failure rather than silently swallowing ───
+
+    def test_activation_script_emits_warning_on_failure(self):
+        self.assertIn("warning: sovran-hosts-update", self.source)
+
+
+if __name__ == "__main__":
+    unittest.main()