fix(iso): seed a real file in /var/log/nvidia-dcgm so it survives squashfs layering

bbc6fb1 chowned the directory correctly but it stayed empty at build
time, and bee_layer_classify (squashfs-layers.sh) only tracks regular
files and symlinks via `find ... -type f -o -type l` — an empty
directory is silently dropped from every layer's rsync --files-from
list and never reaches the built ISO. dcgmi diag's deployment check
still failed with DCGM_FR_FILE_CREATE_PERMISSIONS after rebuilding on
top of that fix because the directory simply didn't exist at boot.

Seed /var/log/nvidia-dcgm/.keep so the directory rides along the
classifier as an implied parent (rsync -a preserves its ownership),
and stop 9999-slim's log sweep from deleting that marker before the
classifier ever sees it.

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

iso/builder/config/hooks/normal/9999-slim.hook.chroot

@@ -27,6 +27,9 @@ rm -rf /var/lib/apt/lists/*
 
 # ── Misc ──────────────────────────────────────────────────────────────────────
 rm -rf /tmp/* /var/tmp/* 2>/dev/null || true
-find /var/log -type f -delete 2>/dev/null || true
+# /var/log/nvidia-dcgm/.keep is intentionally seeded by 9000-bee-setup so an
+# otherwise-empty DCGM_HOME_DIR survives the squashfs layer classifier (which
+# only tracks files/symlinks, not empty directories) — never sweep it up here.
+find /var/log -type f ! -path '/var/log/nvidia-dcgm/.keep' -delete 2>/dev/null || true
 
 echo "=== slim: done ==="