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.

.github/docker/Dockerfile.ci

@@ -69,10 +69,10 @@ RUN curl --retry 5 --retry-delay 5 --retry-connrefused -fsSL "https://nodejs.org
 # The version MUST be passed as a positional arg — bun.sh/install ignores a
 # BUN_VERSION env var, so the old `| BUN_VERSION=x.y.z bash` form silently
 # installed latest on every image rebuild (observed: 1.3.13/1.3.14 drift vs
-# the 1.3.10 devs run locally).
+# the 1.3.10 devs ran locally).
 ENV BUN_INSTALL="/usr/local"
 RUN curl --retry 5 --retry-delay 5 --retry-connrefused -fsSL https://bun.sh/install \
-    | bash -s "bun-v1.3.13"
+    | bash -s "bun-v1.4.0"
 
 # Claude CLI — pinned to an EXACT version, same discipline as the bun pin
 # above. The PTY harness (test/helpers/claude-pty-runner.ts) screen-scrapes