Document that POI events carry an identifier only on the Google provider

[jkasprzyk17]

Problem

onPoiPress gives a placeId on the Google provider and nothing identifying on Apple.
AppleMapProviderAdapter.notifyPoiPress sets placeId: nil
(package/ios/AppleMapProviderAdapter.swift:360-372), because MKMapFeatureAnnotation
exposes no stable public identifier.

Since the default provider is apple on iOS and google on Android
(package/src/providers.ts:4-20), the same <MapView onPoiPress={...} /> gives:

Platform (no provider prop)	Payload
iOS	{ provider: 'apple', coordinate, name?, category, rawCategory? }
Android	{ provider: 'google', coordinate, name, placeId }

The types are honest about this — PoiPressEvent is a discriminated union keyed on
provider (package/src/types/map.ts:24-39), so TypeScript forces narrowing before
placeId can be read, and the README documents the three payload shapes
(README.md:307-311).

What is missing is the consequence, which is what anyone building "tap a POI → look it up"
hits: on Apple there is no identifier to look anything up with, and the fix is to opt into
provider="google" on iOS.

Proposal

Add to the POI section of the README:

• Apple POI events carry no identifier. Correlate by coordinate + name, or use
  provider="google" on iOS if a Place ID is needed on both platforms.
• placeId is a Google Place ID, usable with the Google Places API.
• A note that the default provider differs per platform, so code written against placeId
  needs an explicit provider="google".

The same note belongs on the ApplePoiPressEvent / GooglePoiPressEvent JSDoc in
package/src/types/map.ts:24-39, since that is what shows up in editor tooltips.

Notes

This came out of checking a claim that placeId itself differs between platforms for the
same POI. It does not, as far as the source goes: both platforms forward the Google Maps
SDK's place ID verbatim, with no parsing or normalisation — iOS
package/ios/GoogleMapProviderAdapter.swift:530-546, Android
GoogleMapProviderAdapter.kt:445-456, and straight through JS at
package/src/components/MapView.tsx:206-213. If the two SDKs ever disagreed for one
feature, that would be a Google-side discrepancy and there would be nothing here to change.

Related

• Add native Apple Maps POI detail presentation #35 — native Apple Maps POI detail presentation, i.e. the MapKit-native way to act on an
  Apple POI without having an identifier for it.