Emit the JavaScriptUnownedValue decode overload for @Union

[tsapeta]

Why

@Union only emitted decode(_: borrowing JavaScriptValue, in:). The JavaScriptUnownedValue overload fell back to the JavaScriptDecodable default in expo-modules-jsi, which calls value.copied(in: runtime) before decoding. Every union argument decoded through the argument fast path paid for that copy, even when the matching case was a primitive that can read the borrowed value.

How

The macro now emits both decode overloads, with the same case order.

In the unowned overload, a case whose payload has its own unowned decode reads the borrowed value directly. The macro recognizes those payloads by spelling: Bool, String, the numeric types, ArrayBuffer, and an optional of any of them. Every other case (a record, an array, a dictionary, or any type the macro doesn't recognize, like a typealias or a SharedObject subclass) would copy the value through the protocol default, so these cases share one owning copy instead. The copy is made the first time such a case runs, and the mismatch error reuses it.

Copies per call of the unowned overload:

• 0 when a borrowing case matches before any other case runs.
• At most 1 otherwise, including on a mismatch. That is the same as the default overload this replaces, so the count never goes up.

A type missing from the list only costs that one shared copy; the result is still correct.

No core API change is needed.

Test plan

• Updated the expected expansions in UnionMacroTests. The Input fixture (Point, [Point], [String: Point], Int?) now shows the shared copy for the first three cases and the borrowed value for Int?. Unions with only builtin payloads emit no copy.
• swift test: 105 + 181 tests pass.
• Built the release plugin and swapped it into expo/expo's node_modules:
  • expo-modules-core's MacroUnionTests (a union with a record case, so it goes through the shared copy) compile and pass: 12 tests.
  • expo-sqlite's unions compile, the object file has SQLiteBindValue.decode(_: JavaScriptUnownedValue, in:), and expo-sqlite's 50 unit tests pass (checked on the first revision of this PR).