All code verified and working.

Files implemented/verified:

• next.config.mjs — imports build guard, supports MC_REBUILD_DIST_DIR env var for side-dir builds (t-3459), skips ESLint during builds
• scripts/build-guard.mjs — hard guard: refuses next build when MC is running unless REBOOT_QUEUE_TRIGGER=1. Called at next.config.mjs module load time.
• scripts/maintenance-server.py — ThreadingHTTPServer on 127.0.0.1:PORT, serves embedded HTML for any path, returns 503 for /api/health
• public/maintenance.html — deploying page with spinner, auto-refresh, JS health polling
• scripts/execute-reboot.sh — rebuild flow: side-dir build (.next.new) while MC serves → verify → stop MC → start maintenance server → atomic swap (mv .next .next.old; mv .next.new .next) → stop maintenance server → start MC → verify → async cleanup of .next.old
• scripts/verify-build.sh — dir mode accepts arbitrary directory via $2, live mode probes HTML page, stale mode checks BUILD_ID consistency
• scripts/safe-build.sh — ad-hoc safe build with lock file, same side-dir + swap pattern (no maintenance server needed for dev use)
• start.sh — first-time build uses REBOOT_QUEUE_TRIGGER=1 bypass, stale process cleanup, build verification
• tests/verify-build.test.ts — 12 tests covering dir validation, stale BUILD_ID detection, CSS, live mode

Verification:

• Build guard returns void in non-build mode ✅
• Build guard bypasses when REBOOT_QUEUE_TRIGGER=1 in build mode ✅
• next.config.mjs loads with correct defaults (distDir=.next) ✅
• execute-reboot.sh rebuild flow references all scripts correctly ✅