Rewrite build.sh to support per-device target/subtarget selection.
Adds Asus RT-AX53U (ramips/mt7621), GL-MT6000, upgrades to OpenWrt 25.12.0-rc4,
and auto-detects tar.zst vs tar.xz for Image Builder download.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
First-boot script that configures a Parahub mesh node with zero user
interaction: batman-adv BATMAN_V mesh, dual-band WiFi (private SAE +
public open), firewall zones with guest isolation, SQM 128kbps shaping,
MAC-derived subnets for collision avoidance, and key generation.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
