docs: initial plan for ZeroTier exit gateway switcher

Design: .omp/plans/zerotier-gateway-switching.md — LuCI app + rpcd/ucode backend + zt-gateway-switch shell script for force / graceful-drain gateway switching on OpenWRT.

README.md

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+# luci-app-zt-gateway
+
+A LuCI application for OpenWRT that lets users switch which remote ZeroTier node
+acts as the internet exit gateway for WIBLAN clients (`10.11.13.0/24`).
+
+Users select a region/gateway, choose a switch mode, and the app reconfigures
+kernel routing, conntrack, hotplug scripts, and UCI network routes with minimal
+disruption.
+
+## Switch modes
+
+- **Force** — instant cutover (~1-2s disruption); existing connections break and
+  clients auto-reconnect through the new gateway. Flushes conntrack for the
+  WIBLAN subnet.
+- **Graceful drain** — zero disruption; existing connections keep using the old
+  gateway via fwmark + a drain routing table, while new connections go through
+  the new gateway immediately. Auto-falls-back to a force switch after a
+  configurable timeout if drain does not complete.
+
+## Status
+
+> **Planning / pre-implementation.**
+
+The full design — file structure, UCI config schema, rpcd backend (ucode),
+`zt-gateway-switch` shell script, LuCI frontend, switch logic for both modes,
+persistence model, health checks, rollback/safety, Docker macvlan test harness,
+and build/install instructions — is the source of truth in:
+
+[`docs/.omp/plans/zerotier-gateway-switching.md`](.omp/plans/zerotier-gateway-switching.md)
+
+(also tracked at `.omp/plans/zerotier-gateway-switching.md`)
+
+Implementation (Makefile, ucode backend, switch script, `overview.js`, UCI
+config, ACL, menu entry) lands in subsequent commits against this repository.
+
+## License
+
+Apache-2.0