platform/app: fix nvbandwidth split fallback, blackbox discovery churn, and add blocking sync brackets around load steps

- gpuBandwidthSocketGroups: a single GPU whose NUMA node fails to resolve
  no longer collapses the whole per-socket nvbandwidth split into one
  fallback pass — it now folds into the last resolved group instead,
  preserving isolation for the sockets that did resolve.
- blackbox discoverMarkedTargets: skip mounting/unmounting devices that
  already have a running worker on every 2s discovery tick. This was
  observed hammering the same USB target continuously (mount+unmount
  every ~2s for the whole session) and contending with the worker's own
  sync cycle, plausibly explaining multi-minute sync cycles seen on a
  real crash bundle.
- syncFilesystem now calls syscall.Sync() directly instead of spawning
  /bin/sync per copied file; blackbox mounts removable targets with
  -o sync so writes are durable without relying on the app-level sync as
  the primary mechanism.
- New platform.SetSyncBracketHook / satJob.syncBracket: blocks (with a
  bounded timeout) on blackbox actually reaching removable media right
  before and right after a diagnostic's real load step (nvbandwidth,
  memtester, stress-ng, dcgmi diag, nccl, smartctl/nvme self-test...),
  instead of only firing a fire-and-forget kick after the job's own log
  file is written. A crash mid-load now has durable evidence the load
  started, not just whatever streamed to the RAM-backed export dir before
  blackbox's next scheduled cycle.

Found investigating a real support bundle where blackbox's last
successful sync (19:55:25) predated both the previous job finishing and
the crashing nvbandwidth job starting (19:56:57) — none of the crash
window ever reached durable media.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

iso/builder/auto/config

@@ -38,7 +38,6 @@ lb config noauto \
     --linux-flavours "amd64" \
     --linux-packages "${LB_LINUX_PACKAGES}" \
     --memtest memtest86+ \
-    --hostname "${LB_HOSTNAME}" \
     --iso-volume "${LB_ISO_VOLUME}" \
     --iso-application "EASY-BEE-${BEE_GPU_VENDOR_UPPER:-NVIDIA}" \
     --bootappend-live "boot=live live-media=/dev/disk/by-label/${LB_ISO_VOLUME} live-media-label=${LB_ISO_VOLUME} components video=1920x1080 console=ttyS0,115200n8 console=tty0 loglevel=3 systemd.show_status=1 username=bee user-fullname=Bee modprobe.blacklist=nouveau,snd_hda_intel,snd_hda_codec_realtek,snd_hda_codec_generic,soundcore" \
@@ -46,3 +45,10 @@ lb config noauto \
     --apt-recommends false \
     --chroot-squashfs-compression-type zstd \
     "${@}"
+
+# live-build (1:20230502, bookworm) has no `lb config --hostname` flag: the
+# chroot_hostname helper zeroes /etc/hostname to "localhost.localdomain" for
+# the duration of package installation and restores it afterwards from this
+# includes file, so this is the only supported way to set it.
+mkdir -p config/includes.chroot_before_packages/etc
+echo "${LB_HOSTNAME}" > config/includes.chroot_before_packages/etc/hostname