Prebuilt node-api facade for published addons #496

Let published prebuilds record a neutral node-api library name, provided by a small prebuilt library the host embeds in the app. Addons then never name an engine, and a Hermes rename or a new engine means adding a facade rather than rebuilding addons. Deferred: the RFC in #447 doesn't need this. It depends on the stub generator in #494.

The facade has no code, table or injection step. On Apple it re-exports the Node-API symbols from Hermes, and on Android it's an empty library with a DT_NEEDED on Hermes. Details are in #493, and the spike findings are in this comment: 18 of 18 tests passed on an iOS simulator and an Android emulator, with async recording only node-api and resolving into Hermes.

Steps

• Run a Release build on a physical iPhone, through signing and App Store validation. This is the main remaining risk and should come first.
• Add a way for a module to tell link to keep its exact name. The spike hacks getLibraryName.
• Add a prebuilt facade per backend, built with cmake-rn against a stub of the engine, with a generated re-export list on Apple.
• Make link choose which facade to embed.

Open questions

• Can weak-node-api be built under the node-api name as an alternative backend, so one addon binary works with either?
• Should in-app and from-source addons use the same path, so there's only one to maintain?

node-api becomes a stable ABI contract once prebuilds ship, so settle the name before publishing any.