fix task 38: plug profiling fd leak on setup failure paths

runTraceWithContext and runHeadlessParquet opened cpu/mem/exec-trace
profile files via setupProfiling, then relied on the shutdown watcher
goroutine (registered later) to close them. If any intervening step
(newEventLoop, recorder.Start) returned an error, the watcher was never
registered and the profile fds leaked until process exit. Add a
defer profiling.stop(logln) immediately after setupProfiling so the fds
are released on every return path. profiling.stop is idempotent via
sync.Once so the watcher and defer can both run harmlessly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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