Add fiscalDeviceIdentifier to the POS Device API

[shopifyzach]

Background

Some countries require every POS installation acting as a cash register to be registered with the tax authority under a Shopify-assigned register identifier (for example, Sweden's manufacturing number / tillverkningsnummer), and fiscal compliance extensions must stamp that identifier on every receipt, journal entry, and report — including while the device is offline.

The GraphQL Admin API exposes this value as PointOfSaleDevice.fiscalDeviceIdentifier (API version 2026-10). Today the only way for a POS UI extension to read it is shopify.session.deviceId → build a GID → query pointOfSaleDevice(id:) via the Direct API. That path is online-only, requires an Admin API access scope, costs a network round-trip before the first fiscal receipt can print, and couples the extension to two API versions.

Solution

Adds an optional, static property to the POS Device API:

shopify.device.fiscalDeviceIdentifier?: string;

• Lives on device alongside name and registerName, which already mirrors the Admin API's PointOfSaleDevice.handle — same class of Shopify-assigned device identity data. session describes who/where (shop, user, location, staff); device describes the hardware.
• Same name as the Admin API field, because it is the same value — developers can correlate it across surfaces.
• Static string rather than a signal: it doesn't change for the lifetime of a device registration (same as registerName).
• undefined when the device's location doesn't require fiscal device registration. Only available on API version 2026-10 and later; the host exposes it version-gated (same approach as session.deviceId).

Companion changes: the extensibility host devicePlugin reads it from the POS domain bridge and exposes it for 2026-10+; POS Mobile supplies the value it already holds locally after device registration.

Docs generate from the JSDoc; the checked-in generated_docs_data_v2.json is regenerated in the usual docs(pos): generate … PR rather than here.

🎩

• yarn type-check ✅ · eslint + prettier on the touched files ✅
• The ui-extensions-tester POS factories don't need a change: the property is optional and the default mock device isn't in a fiscal-registration country.
• End-to-end: once the host plugin and POS Mobile changes ship, an extension on api_version = "2026-10" running on a POS device at a Swedish location reads shopify.device.fiscalDeviceIdentifier (e.g. "SH-9IX-7"); on any other device it is undefined; on 2026-07 and earlier the property is absent.

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation