upload-to-s3: fail early if the source file is unreadable

If $src can't be read, `tee < "$src"` fails and set -e aborts -- but the
background sha256/wc readers, already blocked opening their fifos, never get a
writer and leak (hang). Guard with `[[ -r "$src" ]]` before launching them,
which also gives a clear error message.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: trvrb

scripts/upload-to-s3

@@ -26,6 +26,11 @@ main() {
     local dst="${2:?A destination s3:// URL is required as the second argument.}"
     local cloudfront_domain="${3:-}"
 
+    # Fail early if $src is unreadable. Otherwise `tee < "$src"` below fails, but
+    # the background hasher/counter -- already blocked opening their fifos -- get
+    # no writer and hang instead of being cleaned up.
+    [[ -r "$src" ]] || { echo "upload-to-s3: cannot read source file: $src" >&2; exit 1; }
+
     local s3path="${dst#s3://}"
     local bucket="${s3path%%/*}"
     local key="${s3path#*/}"