intercom: handle shutdown→boot race and relay through @intercom/messenger-js-sdk

[productdevbook]

Biggest open pain in `devrnt/react-use-intercom` (376 ⭐, ~462k weekly) plus discussion about migrating to official `@intercom/messenger-js-sdk` (~999k weekly).

Evidence

• `Potential replacement? @intercom/messenger-js-sdk? devrnt/react-use-intercom#676` — "Potential replacement? `@intercom/messenger-js-sdk`?" Potential replacement? @intercom/messenger-js-sdk? devrnt/react-use-intercom#676
• `Calling shutdown one time lose all the IntercomContext reactivity until the page is refreshed devrnt/react-use-intercom#666` — "shutdown loses context reactivity until refresh" 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" Intercom Fails After Login Without Reload devrnt/react-use-intercom#827
• `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
• `Intercom deleted on change to language_override nhagen/react-intercom#60` — "Intercom deleted on change to `language_override`" Intercom deleted on change to language_override nhagen/react-intercom#60
• `Fail to catch errors on unmount nhagen/react-intercom#34` — "Fail to catch errors on unmount" Fail to catch errors on unmount nhagen/react-intercom#34
• `[react-use-intercom] onHide, onShow, onUserEmailSupplied, boot Intercom instance is not initalized yet (NextJS 15) devrnt/react-use-intercom#726` — "Boot Intercom instance is not initialized yet (NextJS 15)" [react-use-intercom] onHide, onShow, onUserEmailSupplied, boot Intercom instance is not initalized yet (NextJS 15) devrnt/react-use-intercom#726
• `The App ID in your code snippet has not been set devrnt/react-use-intercom#812` — "The App ID in your code snippet has not been set" The App ID in your code snippet has not been set devrnt/react-use-intercom#812

Tasks

• Internal listener registry that survives `shutdown` and re-binds on `boot`
• `softReboot()` = `shutdown` + `boot` atomically (used during user switch)
• `identify({ hash })` ⇒ maps to `user_hash` (classic) or `intercom_user_jwt` when `auth: "jwt"` config is set
• Region-aware: `region: "us" | "eu" | "au"` ⇒ right `api_base`
• Optional adapter mode that wraps `@intercom/messenger-js-sdk` underneath (delegate `Intercom()` to their typed helper)
• Expose `onUnreadCountChange(count)` typed callback (replaces #479)
• `isBooted` boolean + `ready: Promise`
• `enabled` flag defaults to `process.env.NODE_ENV === 'production'`

[productdevbook]

Implemented in this commit