Add fiscalDeviceIdentifier to the POS Device API

[shopifyzach]

Background

The GraphQL Admin API exposes a 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.

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.

Checklist

• I have 🎩'd these changes
  • Can only be tophatted once the host plugin and POS Mobile changes ship
• I have updated relevant documentation

[henryStelle]

it isn't available on latest as linked, only the RC