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_signature.go

@@ -61,18 +61,26 @@ var ErrSignatureRequired = errors.New("banger release public key is the placehol
 
 // VerifyBlobSignature checks that sigBase64 is a valid cosign-blob
 // signature over body, made with the private counterpart of
-// BangerReleasePublicKey. cosign's blob signature format is a
-// base64-encoded ASN.1-DER ECDSA signature over SHA256(body) — that's
-// what the package's ecdsa.VerifyASN1 verifies natively.
-//
-// Refuses outright if the embedded public key is still the build-
-// time placeholder, so an unset key can't slip through as
-// "verification disabled."
+// BangerReleasePublicKey.
 func VerifyBlobSignature(body, sigBase64 []byte) error {
-	if isPlaceholderKey(BangerReleasePublicKey) {
+	return VerifyBlobSignatureWithKey(body, sigBase64, BangerReleasePublicKey)
+}
+
+// VerifyBlobSignatureWithKey is VerifyBlobSignature against an
+// explicit PEM-encoded public key. Used by the smoke suite (via
+// `banger update --pubkey-file …`) so an end-to-end update test can
+// trust a locally-generated keypair without rebuilding the binary.
+//
+// Refuses outright if pubKeyPEM is the build-time placeholder so an
+// unset key can't slip through as "verification disabled".
+//
+// cosign's blob signature format is a base64-encoded ASN.1-DER ECDSA
+// signature over SHA256(body) — that's what ecdsa.VerifyASN1 takes.
+func VerifyBlobSignatureWithKey(body, sigBase64 []byte, pubKeyPEM string) error {
+	if isPlaceholderKey(pubKeyPEM) {
 		return ErrSignatureRequired
 	}
-	block, _ := pem.Decode([]byte(BangerReleasePublicKey))
+	block, _ := pem.Decode([]byte(pubKeyPEM))
 	if block == nil {
 		return fmt.Errorf("decode banger release public key: no PEM block")
 	}
@@ -96,15 +104,21 @@ func VerifyBlobSignature(body, sigBase64 []byte) error {
 }
 
 // FetchAndVerifySignature pulls the SHA256SUMS.sig URL from the
-// release, downloads it (capped), and verifies it against
-// sumsBody. Returns nil on a clean pass, or an error describing
-// exactly why verification failed.
+// release, downloads it (capped), and verifies it against sumsBody.
+// Returns nil on a clean pass, or an error describing exactly why
+// verification failed.
 //
 // If release.SHA256SumsSigURL is empty, treat that as "release was
 // not signed" — refuse rather than silently proceeding. v0.1.0
 // requires every release to be cosign-signed; an unsigned release
 // is a manifest publishing bug we'd rather catch loudly.
 func FetchAndVerifySignature(ctx context.Context, client *http.Client, release Release, sumsBody []byte) error {
+	return FetchAndVerifySignatureWithKey(ctx, client, release, sumsBody, BangerReleasePublicKey)
+}
+
+// FetchAndVerifySignatureWithKey is FetchAndVerifySignature against
+// an explicit PEM-encoded public key.
+func FetchAndVerifySignatureWithKey(ctx context.Context, client *http.Client, release Release, sumsBody []byte, pubKeyPEM string) error {
 	if strings.TrimSpace(release.SHA256SumsSigURL) == "" {
 		return fmt.Errorf("release %s has no sha256sums_sig_url; refusing to install an unsigned release", release.Version)
 	}
@@ -115,7 +129,7 @@ func FetchAndVerifySignature(ctx context.Context, client *http.Client, release R
 	if err != nil {
 		return fmt.Errorf("fetch signature: %w", err)
 	}
-	if err := VerifyBlobSignature(sumsBody, sig); err != nil {
+	if err := VerifyBlobSignatureWithKey(sumsBody, sig, pubKeyPEM); err != nil {
 		return fmt.Errorf("verify SHA256SUMS signature: %w", err)
 	}
 	return nil