Design pillar: shutdown() must be reversible — listeners & state survive

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Competitor wrappers all suffer the same lifecycle bug: calling `shutdown()` and then `boot()` again leaves the widget in a broken state. We need a deterministic state machine.

Evidence

• `Calling shutdown one time lose all the IntercomContext reactivity until the page is refreshed devrnt/react-use-intercom#666` — "Calling shutdown one time loses all the IntercomContext reactivity until the page is refreshed" — gold-standard root-cause writeup Calling shutdown one time lose all the IntercomContext reactivity until the page is refreshed devrnt/react-use-intercom#666
• `Intercom Fails After Login Without Reload devrnt/react-use-intercom#827` — "Intercom Fails After Login Without Reload" — 403 "App suspended" on soft reboot Intercom Fails After Login Without Reload devrnt/react-use-intercom#827
• `Fail to catch errors on unmount nhagen/react-intercom#34` — "Fail to catch errors on unmount" — `window.Intercom is not a function` Fail to catch errors on unmount nhagen/react-intercom#34
• `IntercomSettings persist on window object after shutdown nhagen/react-intercom#39` — "IntercomSettings persist on window object after shutdown" IntercomSettings persist on window object after shutdown nhagen/react-intercom#39
• `When the Zendesk component unmounts and then remounts, it no longer functions correctly. B3nnyL/react-zendesk#45` — "Unmount then remount no longer functions correctly" When the Zendesk component unmounts and then remounts, it no longer functions correctly. B3nnyL/react-zendesk#45
• `How to remove the Zendesk Element? B3nnyL/react-zendesk#28` — "How to remove the Zendesk Element" How to remove the Zendesk Element? B3nnyL/react-zendesk#28
• `想问下这个插件里边没有 login 的方法吗，我看官方是有的 tawk/tawk-messenger-react#42` — "no login method" — wrapper can't rebind after switch 想问下这个插件里边没有 login 的方法吗，我看官方是有的 tawk/tawk-messenger-react#42
• `[Feature Request] Hot swap websiteID / reconfigure with new websiteID crisp-im/crisp-sdk-web#46` — "Hot swap websiteID / reconfigure with new websiteID" [Feature Request] Hot swap websiteID / reconfigure with new websiteID crisp-im/crisp-sdk-web#46

Contract — state machine

`idle → loading → ready → shutdown → idle`

1. Listener registry lives at adapter level, not in the client's React/Vue closure. Survives shutdown; re-binds on next boot.
2. `shutdown()` actually removes: script tag, window globals, CSS injected, iframe, registered listeners.
3. `reconfigure(newOpts)` = `shutdown() + load(newOpts)` atomically; no intermediate `idle` observable.
4. `boot()` is idempotent & singleton-by-config-hash. Called twice with same opts ⇒ returns existing handle. Called with different opts ⇒ clean teardown first.
5. `destroy()` is the hard kill (removes all state); `shutdown()` keeps the user session data.

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Implemented in c532a7c