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/manifest.go

@@ -75,15 +75,23 @@ type Release struct {
 // Release.
 const ManifestSchemaVersion = 1
 
-// FetchManifest downloads the release manifest and validates its
-// shape. Returns an error if the server is unreachable, returns
-// non-2xx, exceeds the size cap, or the schema_version is newer
-// than this CLI knows.
+// FetchManifest downloads the release manifest from the embedded
+// canonical URL and validates its shape. Returns an error if the
+// server is unreachable, returns non-2xx, exceeds the size cap, or
+// the schema_version is newer than this CLI knows.
 func FetchManifest(ctx context.Context, client *http.Client) (Manifest, error) {
+	return FetchManifestFrom(ctx, client, manifestURL)
+}
+
+// FetchManifestFrom is FetchManifest against an explicit URL. Used by
+// the smoke suite (via `banger update --manifest-url …`) to drive the
+// updater against a locally-served fake manifest. Production callers
+// stick with FetchManifest.
+func FetchManifestFrom(ctx context.Context, client *http.Client, url string) (Manifest, error) {
 	if client == nil {
 		client = http.DefaultClient
 	}
-	req, err := http.NewRequestWithContext(ctx, http.MethodGet, manifestURL, nil)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {
 		return Manifest{}, err
 	}