fix: prevent Go runtime memory corruption and sandbox halt after snapshot restore

Three root causes addressed:

1. Go page allocator corruption: allocations between the pre-snapshot GC
   and VM freeze leave the summary tree inconsistent. After restore, GC
   reads corrupted metadata — either panicking (killing PID 1 → kernel
   panic) or silently failing to collect, causing unbounded heap growth
   until OOM. Fix: move GC to after all HTTP allocations in
   PostSnapshotPrepare, then set GOMAXPROCS(1) so any remaining
   allocations run sequentially with no concurrent page allocator access.
   GOMAXPROCS is restored on first health check after restore.

2. PostInit timeout starvation: WaitUntilReady and PostInit shared a
   single 30s context. If WaitUntilReady consumed most of it, PostInit
   failed — RestoreAfterSnapshot never ran, leaving envd with keep-alives
   disabled and zombie connections. Fix: separate timeout contexts.

3. CP HTTP server missing timeouts: no ReadHeaderTimeout or IdleTimeout
   caused goroutine leaks from hung proxy connections. Fix: add both,
   matching host agent values.

Also adds UFFD prefetch to proactively load all guest pages after restore,
eliminating on-demand page fault latency for subsequent RPC calls.

envd/internal/api/init.go

@@ -150,15 +150,17 @@ func (a *API) PostInit(w http.ResponseWriter, r *http.Request) {
 		host.PollForMMDSOpts(ctx, a.mmdsChan, a.defaults.EnvVars)
 	}()
 
-	// Close zombie connections from before the snapshot and re-enable
-	// keep-alives. On first boot this is a no-op (no zombie connections).
+	// Safety net: if the health check's postRestoreRecovery didn't run yet
+	// (e.g. PostInit arrived before the first health check), re-enable GC
+	// here. On first boot needsRestore is false so CAS is a no-op.
+	if a.needsRestore.CompareAndSwap(true, false) {
+		a.postRestoreRecovery()
+	}
+	// RestoreAfterSnapshot is idempotent (clears preSnapshot set), and
+	// Start is a no-op if already running.
 	if a.connTracker != nil {
 		a.connTracker.RestoreAfterSnapshot()
 	}
-
-	// Start the port scanner and forwarder if they were stopped by a
-	// pre-snapshot prepare call. Start is a no-op if already running,
-	// so this is safe on first boot and only takes effect after restore.
 	if a.portSubsystem != nil {
 		a.portSubsystem.Start(a.rootCtx)
 	}