update: VMs survive banger update and rollback

Three load-bearing fixes that together let `banger update` (and its
auto-rollback path) restart the helper + daemon without killing
every running VM. New smoke scenarios prove the property end-to-end.

Bug fixes:

1. Disable the firecracker SDK's signal-forwarding goroutine. The
   default ForwardSignals = [SIGINT, SIGQUIT, SIGTERM, SIGHUP,
   SIGABRT] installs a handler in the helper that propagates the
   helper's SIGTERM (sent by systemd on `systemctl stop bangerd-
   root.service`) to every running firecracker child. Set
   ForwardSignals to an empty (non-nil) slice so setupSignals
   short-circuits at len()==0.

2. Add SendSIGKILL=no to bangerd-root.service. KillMode=process
   limits the initial SIGTERM to the helper main, but systemd
   still SIGKILLs leftover cgroup processes during the
   FinalKillSignal stage unless SendSIGKILL=no.

3. Route restart-helper / restart-daemon / wait-daemon-ready
   failures through rollbackAndRestart instead of rollbackAndWrap.
   rollbackAndWrap restored .previous binaries but didn't re-
   restart the failed unit, leaving the helper dead with the
   rolled-back binary on disk after a failed update.

Testing infrastructure (production binaries unaffected):

- Hidden --manifest-url and --pubkey-file flags on `banger update`
  let the smoke harness redirect the updater at locally-built
  release artefacts. Marked Hidden in cobra; not advertised in
  --help.
- FetchManifestFrom / VerifyBlobSignatureWithKey /
  FetchAndVerifySignatureWithKey export the existing logic against
  caller-supplied URL / pubkey. The default entry points still
  call them with the embedded canonical values.

Smoke scenarios:

- update_check: --check against fake manifest reports update
  available
- update_to_unknown: --to v9.9.9 fails before any host mutation
- update_no_root: refuses without sudo, install untouched
- update_dry_run: stages + verifies, no swap, version unchanged
- update_keeps_vm_alive: real swap to v0.smoke.0; same VM (same
  boot_id) answers SSH after the daemon restart
- update_rollback_keeps_vm_alive: v0.smoke.broken-bangerd ships a
  bangerd that passes --check-migrations but exits 1 as the
  daemon. The post-swap `systemctl restart bangerd` fails,
  rollbackAndRestart fires, the .previous binaries are restored
  and re-restarted; the same VM still answers SSH afterwards
- daemon_admin (separate prep): covers `banger daemon socket`,
  `bangerd --check-migrations --system`, `sudo banger daemon
  stop`

The smoke release builder generates a fresh ECDSA P-256 keypair
with openssl, signs SHA256SUMS cosign-compatibly, and serves
artefacts from a backgrounded python http.server.
verify_smoke_check_test.go pins the openssl/cosign signature
equivalence so the smoke release builder can't silently drift.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/updater/verify_smoke_check_test.go

@@ -0,0 +1,54 @@
+package updater
+
+import (
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+// TestVerifyBlobSignatureWithOpenSSL is a confidence test for the
+// smoke release-builder path: openssl's `dgst -sha256 -sign` produces
+// the exact same encoding cosign emits for blob signatures (base64
+// ASN.1 ECDSA over SHA256(body)). If this ever stops verifying, the
+// smoke update scenarios will silently skip the signature check —
+// catching it here avoids a heisenbug in scripts/smoke.sh.
+func TestVerifyBlobSignatureWithOpenSSL(t *testing.T) {
+	if _, err := exec.LookPath("openssl"); err != nil {
+		t.Skip("openssl not on PATH")
+	}
+	dir := t.TempDir()
+	keyPath := filepath.Join(dir, "cosign.key")
+	pubPath := filepath.Join(dir, "cosign.pub")
+	bodyPath := filepath.Join(dir, "body.txt")
+	sigPath := filepath.Join(dir, "body.sig")
+
+	mustRun := func(name string, args ...string) {
+		t.Helper()
+		out, err := exec.Command(name, args...).CombinedOutput()
+		if err != nil {
+			t.Fatalf("%s %v: %v\n%s", name, args, err, string(out))
+		}
+	}
+
+	mustRun("openssl", "ecparam", "-name", "prime256v1", "-genkey", "-noout", "-out", keyPath)
+	mustRun("openssl", "ec", "-in", keyPath, "-pubout", "-out", pubPath)
+	mustRun("sh", "-c", "printf 'banger smoke release sums\n' > "+bodyPath)
+	mustRun("sh", "-c", "openssl dgst -sha256 -sign "+keyPath+" "+bodyPath+" | base64 -w0 > "+sigPath)
+
+	body := readFile(t, bodyPath)
+	sig := readFile(t, sigPath)
+	pub := readFile(t, pubPath)
+
+	if err := VerifyBlobSignatureWithKey(body, sig, string(pub)); err != nil {
+		t.Fatalf("VerifyBlobSignatureWithKey: %v", err)
+	}
+}
+
+func readFile(t *testing.T, p string) []byte {
+	t.Helper()
+	out, err := exec.Command("cat", p).Output()
+	if err != nil {
+		t.Fatalf("read %s: %v", p, err)
+	}
+	return out
+}