fix: eliminate browser and process-cleanup test flakes

Pin every CI surface to Bun 1.4.0 to avoid extra-stdio finalizers closing
reused live sockets. Add an isolated GC/listener regression that fails on
Bun 1.3.13, and prevent coordinated rollback to an affected CI runtime.

Check renderer cleanup against the render's own staging directory so
concurrent renders cannot invalidate the assertion. Make the no-pgrep
process-tree walk tolerate disappearing /proc entries, and synchronize
its test fixture through child readiness and pipe EOF instead of sleeps.

Validation: 9,157 passed, 31 skipped, zero failures across 556 files with
retries disabled. Build, all-host generation freshness, and skill checks
passed. All three races have failing-before/passing-after regressions.

setup

@@ -675,9 +675,9 @@ fi
 # potentially leaving Playwright cache locks held. macOS ships no setsid
 # binary, so a portable group-kill isn't available; walk `pgrep -P` children
 # depth-first instead (pgrep exists on macOS and Linux). Falls back to a
-# plain kill of the root pid when pgrep is unavailable.
+# /proc walk where available, otherwise a plain kill of the root pid.
 _kill_tree() {
-  local pid="$1" child
+  local pid="$1" child stat_file stat_line proc_state proc_parent proc_rest
   if command -v pgrep >/dev/null 2>&1; then
     for child in $(pgrep -P "$pid" 2>/dev/null); do
       _kill_tree "$child"
@@ -686,8 +686,15 @@ _kill_tree() {
     # debian-slim and git-bash ship no pgrep: walk /proc for children. The
     # comm field "(name)" may contain spaces and parens, so strip through the
     # LAST closing paren (proc(5)) before reading the ppid (second field after it).
-    for child in $(awk -v p="$pid" '{ s=$0; sub(/^.*\) /, "", s); split(s, f, " "); if (f[2]==p) print $1 }' /proc/[0-9]*/stat 2>/dev/null); do
-      _kill_tree "$child"
+    # A process may exit after glob expansion. One awk over every stat file
+    # aborts at that missing entry and never visits the remaining live children.
+    for stat_file in /proc/[0-9]*/stat; do
+      IFS= read -r stat_line 2>/dev/null < "$stat_file" || continue
+      child="${stat_line%% *}"
+      IFS=' ' read -r proc_state proc_parent proc_rest <<< "${stat_line##*) }"
+      if [ "$proc_parent" = "$pid" ]; then
+        _kill_tree "$child"
+      fi
     done
   fi
   kill -9 "$pid" 2>/dev/null || true