build_sdk: also reuse an SDK pre-placed under ~/Downloads

The Orin came with the SDK source at ~/Downloads/OrbbecSDK_v2; add it to the
reuse candidates so a clean install finds the build there instead of cloning a
second copy.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/build_sdk.sh

@@ -70,7 +70,9 @@ inject_recorder() {
 # missing there — never disturb the library build.
 for candidate in "${OB_SDK_ROOT:-}" \
                  "$HOME/OrbbecSDK_v2" \
-                 "$HOME/OrbbecSDK_v2_jetson"; do
+                 "$HOME/OrbbecSDK_v2_jetson" \
+                 "$HOME/Downloads/OrbbecSDK_v2" \
+                 "$HOME/Downloads/OrbbecSDK_v2_jetson"; do
     [[ -z "$candidate" ]] && continue
     if b=$(find_build "$candidate" 2>/dev/null); then
         echo ">> SDK deja bati, reutilise: $candidate" >&2