feat(update): pre-flight gates + deeper healthcheck + auto-rollback

Phase 8 of v0.3. Tightens the update lifecycle on both ends.

Pre-flight (apply.go, before any download):
- Free-space check on the passive partition: image size + 10% headroom must
  be available. Uses statfs(2) via the new pkg/partition.FreeBytes /
  HasFreeSpaceFor helpers (tests cover happy path, tiny request, huge
  request, missing path). Catches corrupted-FS and shrunk-partition cases
  before we destroy the existing slot data.
- Node-block-label check: refuses if the local K8s node carries the
  updates.kubesolo.io/block=true label. New pkg/health.CheckNodeBlocked
  shells out to kubectl per the project's zero-deps stance. Silently bypassed
  when no kubeconfig is reachable (air-gap case). Skipped by --force.

Healthcheck (extended via new pkg/health/extended.go + preflight.go):
- CheckKubeSystemReady waits until every kube-system pod has held the Running
  phase for >= N seconds (default 30). Catches "started ok, will crash-loop"
  bugs that a single-shot phase check misses.
- CheckProbeURL fetches an operator-supplied URL; 200 = pass. Wired through
  update.conf as healthcheck_url= and cloud-init updates.healthcheck_url.
- CheckDiskWritable writes/fsyncs/reads a 1-KiB probe under /var/lib/kubesolo.
  Always runs in healthcheck so a wedged data partition fails fast.
- pkg/health.Status grows KubeSystemReady, ProbeURL, DiskWritable booleans.
  Optional checks default to true in RunAll() so they don't block when
  unconfigured. health_test.go updated to the new 6-field shape.

Auto-rollback (healthcheck.go):
- state.UpdateState gains HealthCheckFailures (consecutive post-Activated
  failures). Reset on a clean pass.
- --auto-rollback-after N (also auto_rollback_after= in update.conf) triggers
  env.ForceRollback() when the failure count reaches the threshold. State
  transitions to RolledBack with a descriptive LastError. The command still
  exits with the healthcheck error; the operator/init is expected to reboot.
- Only fires while Phase == Activated. Doesn't second-guess a long-stable
  system that happens to fail one healthcheck.

config / opts / cloud-init plumbing:
- update.conf gains healthcheck_url= and auto_rollback_after= keys.
- New CLI flags: --healthcheck-url, --auto-rollback-after, --kube-system-settle.
- cloud-init full-config.yaml documents the new updates: subfields.

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

update/pkg/partition/freespace_test.go

@@ -0,0 +1,44 @@
+package partition
+
+import "testing"
+
+func TestFreeBytesReturnsNonZeroOnTempDir(t *testing.T) {
+	b, err := FreeBytes(t.TempDir())
+	if err != nil {
+		t.Fatalf("FreeBytes: %v", err)
+	}
+	// On any sane test runner the temp filesystem has more than 1 KiB free.
+	if b < 1024 {
+		t.Errorf("FreeBytes = %d, want > 1024 on /tmp", b)
+	}
+}
+
+func TestFreeBytesNonExistentPath(t *testing.T) {
+	_, err := FreeBytes("/this/path/does/not/exist/at/all")
+	if err == nil {
+		t.Error("expected error for missing path, got nil")
+	}
+}
+
+func TestHasFreeSpaceForRejectsHugeRequest(t *testing.T) {
+	// Request 1 PiB with 10% headroom on /tmp — no test runner has that
+	// much free, so this should consistently report not-enough.
+	avail, ok, err := HasFreeSpaceFor(t.TempDir(), 1<<50, 10)
+	if err != nil {
+		t.Fatalf("HasFreeSpaceFor: %v", err)
+	}
+	if ok {
+		t.Errorf("expected insufficient space for 1PiB, got avail=%d ok=true", avail)
+	}
+}
+
+func TestHasFreeSpaceForAcceptsSmallRequest(t *testing.T) {
+	// 1 KiB with 10% headroom = 1.1 KiB. Any temp dir has this.
+	_, ok, err := HasFreeSpaceFor(t.TempDir(), 1024, 10)
+	if err != nil {
+		t.Fatalf("HasFreeSpaceFor: %v", err)
+	}
+	if !ok {
+		t.Error("expected sufficient space for 1KiB on /tmp")
+	}
+}