fix(neuron): run service as neuron user, not cortex

neuron and cortex are independent packages installable on different
hosts. Having neuron run under a 'cortex' system user implied a
shared identity that doesn't exist. Give neuron its own user/group.

- New data/neuron-sysusers.conf declares the neuron user/group with
  home /var/lib/neuron.
- systemd unit User/Group changed to neuron.
- Spec file attrs, explicit Provides, and %sysusers_create_compat
  updated to reference the neuron user.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

data/neuron.service

@@ -8,8 +8,8 @@ Type=simple
 ExecStart=/usr/bin/neuron --config /etc/neuron/neuron.toml
 Restart=on-failure
 RestartSec=5
-User=cortex
-Group=cortex
+User=neuron
+Group=neuron
 
 [Install]
 WantedBy=multi-user.target