[FIX] script qemu: Arch networking (network-config key = eth0)

Arch VMs booted (after the Secure Boot fix) but eth0 stayed DOWN and
cloud-init never finished -> no IP, no SSH. Cause found in the guest:
cloud-init's Arch renderer writes the systemd-networkd [Match] Name from
the ethernet KEY of the v2 config (ignoring "match:"), so our key
"primary" produced "Name=primary" which matches no interface (Arch's NIC
is eth0). Rename the key to "eth0": Arch now renders "Name=eth0" and
DHCPs; Debian/Fedora/Ubuntu still use "match: name: e*" (verified Debian
redeploy: enp1s0 UP with an IP), so no regression.

Arch is now fully working end to end: boot, DHCP IP, SSH by key, user
erplibre in sudo, passwordless sudo, console login erplibre/erplibre.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

script/qemu/deploy_qemu.py

@@ -823,10 +823,13 @@ def build_cloud_config(
 # network-config (cloud-init v2) : DHCP sur toute interface « e* ». Les images
 # Debian genericcloud ne configurent pas toujours le réseau sans ça (le NIC
 # reste down -> pas d'IP), contrairement à Ubuntu. Inoffensif pour Ubuntu.
+# La clé est « eth0 » car le renderer cloud-init d'Arch utilise la CLÉ comme
+# nom d'interface (en ignorant « match ») et l'image Arch nomme son NIC eth0 ;
+# Debian/Fedora/Ubuntu utilisent bien « match: name: e* » (leur en*).
 NETWORK_CONFIG = (
     "version: 2\n"
     "ethernets:\n"
-    "  primary:\n"
+    "  eth0:\n"
     "    match:\n"
     '      name: "e*"\n'
     "    dhcp4: true\n"