fix(xray): synchronize process status checks

[dr-hoseyn]

Core.Started reads exec.Cmd.ProcessState while Cmd.Wait writes it, and it reads the process pointer while lifecycle methods replace it. Health checks and API status requests can therefore race with process exit, stop, or restart.

This change protects process status checks with the core mutex and uses the existing waitDone channel to observe process exit instead of reading ProcessState. Start and Stop call a lock-held helper so they do not recursively acquire the mutex. Regression tests use a real child process to cover exit, concurrent polling during stop/replacement, and duplicate-start rejection without requiring an installed Xray binary.

Validation:

• go test ./backend/xray -run '^TestCoreStarted' -count=25 -timeout=180s — passed
• go vet ./backend/xray — passed
• git diff --check — passed
• Full local Xray package run reaches the existing integration tests, then fails because /usr/local/bin/xray is unavailable on this Windows host.
• Local -race execution is unavailable because this host has no C compiler. The PR adds a focused Linux CI race check; GitHub currently marks the fork workflow as action_required, pending maintainer approval.

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