test: lock in the PostgreSQL 18.6 container criteria (E00-S03-T01)

- compose-config.test.mjs: assertDbService requires the pinned
  postgres:18.6-bookworm image; new docker-gated real-stack probe starts the
  stack and asserts SHOW server_version exposes 18.6; mutation probe proves
  reverting to a floating major tag fails the criterion; parser probe updated
- build-targets.test.mjs: db-service mutation fixture updated to the pinned
  image tag

tests/compose-config.test.mjs

@@ -180,8 +180,9 @@ function assertDbService(compose) {
   assert.ok(db, 'compose.yaml must declare a "db" service (docker compose up -d starts the database)');
   assert.equal(
     db.image,
-    'postgres:18-bookworm',
-    'the "db" service must use the committed PostgreSQL 18 image (postgres:18-bookworm)',
+    'postgres:18.6-bookworm',
+    'the "db" service must use the committed PostgreSQL 18.6 image (postgres:18.6-bookworm — E00-S03-T01 ' +
+      'pins the exact minor so the database container exposes the expected PostgreSQL version)',
   );
   const env = db.environment ?? {};
   assert.equal(env.POSTGRES_DB, '${POSTGRES_DB:-eppp}', 'db must set POSTGRES_DB (with a default)');
@@ -640,6 +641,43 @@ test('a database fixture survives docker compose restart and recreate (named db-
   }
 });
 
+test('the database container exposes the expected PostgreSQL version (18.6)', { skip: !DOCKER_COMPOSE || !DOCKER_DAEMON }, () => {
+  // E00-S03-T01 acceptance criteria: a PostgreSQL 18.6 container is used as
+  // the database AND the container exposes the expected PostgreSQL version.
+  // The committed image tag is pinned to postgres:18.6-bookworm (asserted
+  // statically above); this probe starts the stack and asks the running
+  // server itself (`SHOW server_version`) — the issue's test plan ("start the
+  // container and confirm PostgreSQL 18.6") — so the version the container
+  // actually exposes is verified end to end, not just the declared tag.
+  const up = run('docker', ['compose', 'up', '-d'], { cwd: REPO_ROOT });
+  assert.equal(
+    up.status,
+    0,
+    `"docker compose up -d" must exit 0:\n${(up.stdout || '')}\n${(up.stderr || '')}`.trim(),
+  );
+  try {
+    waitForDbHealthy();
+    const version = run(
+      'docker',
+      ['compose', 'exec', '-T', 'db', 'psql', '-U', 'eppp', '-d', 'eppp', '-tA', '-c', 'SHOW server_version;'],
+      { cwd: REPO_ROOT, timeout: 60_000 },
+    );
+    assert.equal(
+      version.status,
+      0,
+      `"SHOW server_version" must succeed against the running db container:\n` +
+        `${(version.stdout || '')}\n${(version.stderr || '')}`.trim(),
+    );
+    assert.match(
+      version.stdout.trim(),
+      /^18\.6\b/,
+      `the running db container must expose PostgreSQL 18.6 (SHOW server_version got: "${version.stdout.trim()}")`,
+    );
+  } finally {
+    run('docker', ['compose', 'down'], { cwd: REPO_ROOT });
+  }
+});
+
 // ---------------------------------------------------------------------------
 // Non-vacuous probes — the assertions above really do fail on violations
 // ---------------------------------------------------------------------------
@@ -648,7 +686,7 @@ test('the YAML parser reads the committed structure (non-vacuous parser probe)',
   const parsed = parseYaml(`
 services:
   db:
-    image: postgres:18-bookworm
+    image: postgres:18.6-bookworm
     environment:
       POSTGRES_DB: eppp
     ports:
@@ -665,7 +703,7 @@ services:
       db:
         condition: service_healthy
 `);
-  assert.equal(parsed.services.db.image, 'postgres:18-bookworm');
+  assert.equal(parsed.services.db.image, 'postgres:18.6-bookworm');
   assert.equal(parsed.services.db.environment.POSTGRES_DB, 'eppp');
   assert.deepEqual(parsed.services.db.ports, ['5432:5432']);
   assert.match(parsed.services.db.healthcheck.test, /pg_isready/);
@@ -682,6 +720,18 @@ test('removing the db service makes the database criterion fail (mutation probe)
   assert.throws(() => assertDbService(compose), /"db" service/);
 });
 
+test('reverting the db image to a floating major tag fails the PostgreSQL 18.6 criterion (mutation probe)', () => {
+  // E00-S03-T01 pins the exact 18.6 minor (postgres:18.6-bookworm). A
+  // floating major tag (postgres:18-bookworm / postgres:18) would not
+  // guarantee the container exposes PostgreSQL 18.6, so reverting to one must
+  // fail the pinned-image assertion.
+  const text = read(COMPOSE_PATH);
+  const mutated = text.replace('postgres:18.6-bookworm', 'postgres:18-bookworm');
+  assert.notEqual(mutated, text, 'the mutation must actually revert the pinned image tag');
+  const compose = parseYaml(mutated);
+  assert.throws(() => assertDbService(compose), /18\.6/);
+});
+
 test('removing the app service makes the application criterion fail (mutation probe)', () => {
   const text = read(COMPOSE_PATH);
   const withoutApp = text.replace(/^  app:\n(?:    .*\n?)*/m, '');