test(linux): avoid buffered serve readiness detection

config/docker/headless-serve-shutdown/run-signal-case.sh

@@ -54,10 +54,10 @@ app_pid=$!
 app_start_ticks=$(awk '{print $22}' "/proc/$app_pid/stat")
 
 # The inner shell expands its positional parameters.
-# shellcheck disable=SC2016
+# shellcheck disable=SC1083,SC2016
 ready_line=$(timeout "$startup_timeout_seconds" bash -c '
   tail --pid="$1" -n +1 -F "$2" 2>/dev/null \
-    | sed -n 's/^[^{]*//p' \
+    | sed -u -n 's/^[^{]*//p' \
     | jq --unbuffered -Rnc '\''first(inputs | fromjson? | select(.type == "orca_server_ready" and .schemaVersion == 1))'\''
 ' bash "$app_pid" "$stdout_log" || true)
 if [[ -z "$ready_line" ]]; then

config/scripts/headless-serve-shutdown-workflow.test.mjs

@@ -5,6 +5,7 @@ import { describe, expect, it } from 'vitest'
 
 const workflow = parse(readFileSync('.github/workflows/pr.yml', 'utf8'))
 const headlessLinuxGuide = readFileSync('docs/reference/headless-linux-server.md', 'utf8')
+const signalCase = readFileSync('config/docker/headless-serve-shutdown/run-signal-case.sh', 'utf8')
 
 function readSystemdUnitBlocks(doc, unitName) {
   const escapedUnitName = unitName.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
@@ -65,6 +66,10 @@ describe('headless serve shutdown PR gate', () => {
     expect(steps.indexOf(appImageShutdownStep)).toBeGreaterThan(steps.indexOf(launcherShutdownStep))
   })
 
+  it('keeps the readiness parser line-buffered', () => {
+    expect(signalCase).toContain("| sed -u -n 's/^[^{]*//p'")
+  })
+
   it('keeps owned Xvfb alive during the documented systemd graceful stop', () => {
     const serveUnits = readSystemdUnitBlocks(headlessLinuxGuide, 'orca-serve.service')
     const ownedXvfbUnits = serveUnits.filter((unit) => !/^Environment=DISPLAY=/m.test(unit))