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/cli/daemon_lifecycle_test.go

@@ -178,7 +178,15 @@ func TestRenderRootHelperSystemdUnitIncludesRequiredCapabilities(t *testing.T) {
 
 	for _, want := range []string{
 		"ExecStart=/usr/local/bin/bangerd --root-helper",
+		// Both directives are load-bearing for "VM survives helper
+		// restart": KillMode=process limits the initial SIGTERM to
+		// the helper main, SendSIGKILL=no disables the SIGKILL
+		// escalation. The helper itself does the cgroup reparent
+		// (see roothelper.reparentToBangerFCCgroup) — without
+		// that, even these directives leave firecracker exposed to
+		// systemd's stop-time cleanup.
 		"KillMode=process",
+		"SendSIGKILL=no",
 		"Environment=TMPDIR=/run/banger-root",
 		"NoNewPrivileges=yes",
 		"PrivateTmp=yes",