Coerce @AppStorage values across primitive types instead of crashing

[vincentborko]

Motivation

On iOS, @AppStorage coerces between primitive types: you can store a value as a Double and read it back as a String, or vice versa. On Android the same code crashes (#317) — e.g. store "1.0" under a key, then read it through a Double-typed @AppStorage:

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Number

Root cause

AppStorage<Value>.trackState() read the stored object with:

value = object as? Value

Under Kotlin's type erasure Value is unknown at runtime, so object as? Value is a no-op: it neither coerces the value nor rejects a mismatched type. A String stored value flowed unchanged into a Double-typed property and crashed a later consumer.

The bridged AppStorageSupport doesn't have this problem because it captures a per-type get closure at init (e.g. { $1 as? Double ?? $0.double(forKey: key) }) using UserDefaults' coercing typed accessors. The generic AppStorage<Value> has only one generic initializer, so it has no such per-type dispatch point — but its _wrappedValue still holds a concrete runtime value.

Fix

Dispatch on the concrete runtime type of _wrappedValue and read through the matching typed UserDefaults accessor, mirroring AppStorageSupport's coercion. Both the initial read and the change-listener read now go through a single storedValue(from:store:) helper. Behavior is otherwise unchanged: an absent key still returns nil (seeding the default), and a deserializer, when present, still takes priority.

Testing

New AppStorageTests (runs under Robolectric via the transpiled test path):

• testAppStorageCoercesStringToDouble — stores "1.0", reads a Double, expects 1.0.
• testAppStorageCoercesIntToDouble — stores 3, reads a Double, expects 3.0.

Verified the tests are a real regression guard: against the pre-fix object as? Value both fail under Robolectric (AssertionError: 1.0 != 1.0 — String vs Double — and 3 != 3.0), and pass with the fix. Full transpiled SkipUI suite green (94 tests, 0 failures).