Speed up VM create with work seeds

Beat VM create wall time without changing VM semantics.

Generate a work-seed ext4 sidecar during image builds and rootfs rebuilds, then clone and resize that seed for each new VM instead of rebuilding /root from scratch. Plumb the new seed artifact through config, runtime metadata, store state, runtime-bundle defaults, doctor checks, and default-image reconciliation so older images still fall back cleanly.

Add a daemon TAP pool to keep idle bridge-attached devices warm, expose stage timing in lifecycle logs, add a create/SSH benchmark script plus Make target, and teach verify.sh that tap-pool-* devices are reusable capacity rather than cleanup leaks.

Validated with go test ./..., make build, ./verify.sh, and make bench-create ARGS="--runs 2".

verify.sh

@@ -38,6 +38,11 @@ firecracker_running() {
   [[ "$cmdline" == *firecracker* && "$cmdline" == *"$api_sock"* ]]
 }
 
+pooled_tap() {
+  local tap="$1"
+  [[ "$tap" == tap-pool-* ]]
+}
+
 wait_for_ssh() {
   local guest_ip="$1"
   local deadline="$2"
@@ -228,8 +233,12 @@ if ./banger vm show "$VM_NAME" >/dev/null 2>&1; then
   exit 1
 fi
 if ip link show "$TAP" >/dev/null 2>&1; then
-  log "tap still exists: $TAP"
-  exit 1
+  if pooled_tap "$TAP"; then
+    log "tap returned to idle pool: $TAP"
+  else
+    log "tap still exists: $TAP"
+    exit 1
+  fi
 fi
 if [[ -d "$VM_DIR" ]]; then
   log "vm dir still exists: $VM_DIR"