WP-13-build_system

Engineering build system: one-command builds (Linux build.sh / Windows build.ps1 / make all), reproducible builds (byte-identical ELF across clean builds, deterministic SRC zip), ARM64 + RISC-V cross-compilation with live-verified bring-up shells, content-addressed compile cache (3.2x), multi-configuration builds (.config / menuconfig / CONFIG_TUI=n boots 216 commands), CI pipeline, BUILD.md guide. Gates on this ISO: 0 errors 0 warnings, WP-12 E2E 14/14, regression 28/28, dhtest + cryptotest, SSH E2E 5/5, HTTPS E2E 7/7, BIOS + UEFI (233 commands).