Avoid unnecessary Unity reimport of generated proto files (#6163)

Stage proto outputs in a temp directory and rsync with --checksum
to preserve timestamps on unchanged files. Previously, rm -rf on
the output directory forced Unity to reimport all protos (~7 min
of script recompilation) even when nothing changed.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

scripts/build_protos.sh

@@ -35,8 +35,13 @@ SUBDIRS=(
 /bin/echo "Building C# protobuf sources..."
 bazel build "${TARGETS[@]}"
 
-/bin/echo "Copying generated .cs files to $OUTPUT_DIR..."
-rm -rf "$OUTPUT_DIR"
+/bin/echo "Syncing generated .cs files to $OUTPUT_DIR..."
+
+# Stage new files in a temp directory, then rsync to preserve timestamps
+# on unchanged files. This avoids triggering Unity reimport when protos
+# haven't changed, saving ~7 minutes of script recompilation.
+STAGING_DIR=$(mktemp -d)
+trap "rm -rf '$STAGING_DIR'" EXIT
 
 for i in "${!TARGETS[@]}"; do
     target="${TARGETS[$i]}"
@@ -50,9 +55,14 @@ for i in "${!TARGETS[@]}"; do
     name="${target_path##*:}"
     bin_dir="$REPO_ROOT/bazel-bin/$pkg/$name"
 
-    dest_dir="$OUTPUT_DIR/$subdir"
+    dest_dir="$STAGING_DIR/$subdir"
     mkdir -p "$dest_dir"
     find "$bin_dir" -name "*.cs" -exec cp {} "$dest_dir/" \;
 done
 
+# Sync only changed files and delete removed ones; --checksum compares
+# content not timestamps so unchanged files keep their original mtime.
+mkdir -p "$OUTPUT_DIR"
+rsync -rc --delete "$STAGING_DIR/" "$OUTPUT_DIR/"
+
 /bin/echo "Done. Generated C# proto sources in $OUTPUT_DIR"