feat: kit-busrouter v2.0 — complete Synology + GL fleet router platform

Includes:
- package/ GL-XE3000 kit-busrouter (opkg)
- scripts/provision.sh (GL) and provision-synology.sh (Synology)
- syno-balance/ — new WAN balancer replacing aiwanbal (SmartWAN adapter)
- kit-connect/ — unified connectivity SPK (Tailscale + reverse SSH)
- docs/deployment/synology-rt2600ac-checklist.md — 62-point checklist
- docs/provisioning/device-identity.md — fleet identity spec
- docs/pilot/checklist.md — field pilot validation
- x4078_20260721.dss — reference config backup

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

syno-balance/bin/lib-decide.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+# lib-decide.sh — PURE hysteresis + weight-formula decision helpers.
+# No side effects: args in, stdout/exit-code out. POSIX/ash-safe.
+
+# Weight for WAN A given scoreA scoreB:
+#   50 + 2*(a-b), rounded to nearest 10, clamped to 20..80.
+# e.g. wan_weight 70 70 -> 50 ; wan_weight 80 60 -> 90 -> clamp 80.
+wan_weight() {
+	awk -v a="$1" -v b="$2" 'BEGIN{
+		w = 50 + (a-b)*2
+		w = int((w+5)/10)*10       # round to nearest 10 (works for w>=-5; clamp handles low end)
+		if (w < 20) w = 20
+		if (w > 80) w = 80
+		print w }'
+}
+
+# Exit 0 (changed) if |target-current| >= 10, else exit 1 (within hysteresis band).
+weight_changed() {
+	[ "$(( $2 > $1 ? $2 - $1 : $1 - $2 ))" -ge 10 ]
+}
+
+# Exit 0 if bad_cycles >= threshold (time to switch SIM), else exit 1.
+sim_should_switch() {
+	[ "$1" -ge "$2" ]
+}
+
+# Exit 0 (gateway alive) if at least 2 of 3 probes pass (each arg: 1=pass 0=fail).
+gw_verdict() {
+	[ "$(( $1 + $2 + $3 ))" -ge 2 ]
+}