fix(doorman): stdbuf cannot exec shell functions — buffer inside decode_stream [#355]

Deployment rehearsal on ultix-field 2026-09-02 caught it live: the
listener showed active but decode_stream never ran (stdbuf exec's
binaries only). Line-buffering moved inside decode_stream; the od
stage keeps its own stdbuf.

Rehearsal note: https://projects.knownelement.com/issues/355

bin/doorman.sh

@@ -61,7 +61,10 @@ usage() {
 #     behavior); Enter on an empty buffer emits nothing (defensive fix).
 # ---------------------------------------------------------------------------
 decode_stream() { # decode_stream <width> — od -t u1 records on stdin -> IDs
-    awk -v w="$1" '
+    # stdbuf exec's binaries only — it can NEVER wrap this function, so
+    # line-buffering happens here inside (piped awk block-buffers otherwise).
+    # shellcheck disable=SC2016  # awk program is single-quoted by design; '"$SELF"' re-opens quotes deliberately
+    stdbuf -oL awk -v w="$1" '
     {
         if (NF != w) { next }
         ti = w - 7; ci = w - 5; vi = w - 3
@@ -143,7 +146,7 @@ stream_device() {
     log "listening on $dev (record width $WIDTH)"
     # stdbuf: od/awk block-buffer when piped; scans must not lag behind
     stdbuf -oL od -v -A n -t u1 -w"$WIDTH" "$dev" 2>/dev/null \
-        | stdbuf -oL decode_stream "$WIDTH" \
+        | decode_stream "$WIDTH" \
         | while read -r bid; do
             process_scan "$bid"
         done