refactor(linux): switch to DynamicUser=yes, drop install-time user creation

AUR installs never call `numa install` — PKGBUILD drops the unit straight
into /usr/lib/systemd/system and the user runs `systemctl enable numa`.
With User=numa the Rust installer's useradd code never fires there,
breaking Arch out of the box.

DynamicUser=yes sidesteps packaging entirely — systemd allocates a
transient UID per start and remaps StateDirectory ownership (including
legacy root-owned trees) automatically. Works on any modern systemd.

Drops the ensure_numa_user_linux/chown helpers plus NUMA_USER; the
unit file alone now captures the privilege-drop story.

numa.service

@@ -9,14 +9,14 @@ ExecStart={{exe_path}}
 Restart=always
 RestartSec=2
 
-User=numa
-Group=numa
+# Transient system user per start; no PKGBUILD/sysusers setup required.
+# systemd remaps the StateDirectory ownership to the dynamic UID on each
+# launch, including legacy root-owned trees from pre-drop installs.
+DynamicUser=yes
 
 AmbientCapabilities=CAP_NET_BIND_SERVICE
 CapabilityBoundingSet=CAP_NET_BIND_SERVICE
 
-# StateDirectory maps to crate::data_dir() default on Linux (/var/lib/numa).
-# systemd auto-creates + chowns on every start, fixing legacy root-owned trees.
 StateDirectory=numa
 StateDirectoryMode=0750
 ConfigurationDirectory=numa