cask/uninstall: skip signalling other users' processes - #24082

uninstall signal: sends the signal to all PIDs for the bundle ID in one
Process.kill call. If another user's PID comes first, Errno::EPERM
stops the current user's app from being signalled while its files are
still removed. A TODO asked to check the owner of each PID.

Now only PIDs whose real UID matches Process.uid are signalled, and
other users' PIDs or PIDs with an unknown owner are skipped with a
warning. I used the real UID and not the effective UID because the kernel
checks the target's real (or saved) UID when it decides if a signal is
allowed.

The UID comes from proc_bsdshortinfo via a new LibProc.real_uid, which
LibProc.parent_pid already reads, so no extra process is spawned.
Behaviour is unchanged when all PIDs are the current user's. I added tests
for other users' PIDs, for an unknown owner, and for LibProc.real_uid.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

AI (Claude Code) was used to review the existing diff, and draft this PR description.
I reviewed its output and verified the changes with the targeted spec and brew lgtm.