Cross-platform observation delivery + WebAssembly build support

[0xLeif]

Makes AppState's reactivity work everywhere Swift's Observation runs, and unblocks WebAssembly.

#150 — observation delivery off Apple

The Observation framework is cross-platform, but AppState only delivered changes on Apple: state setters write to the untracked Cache, and the Combine bridge consume(object: cache) (Apple-only) called notifyChange(). On Linux/Windows/wasm there was no bridge, so withObservationTracking { … } onChange: never fired.

Now every mutation path (State / StoredState / FileState / SyncState / SecureState / DependencySlice) calls notifyChange() directly on all platforms, and the Combine cache bridge is removed so Apple still fires exactly once. Observation tests are un-gated to run cross-platform (Apple-only SecureState/SyncState assertions guarded individually) — CI now exercises observation on Linux.

#149 — WebAssembly build

Combine-gated #if !os(Linux) && !os(Windows) guards (import Combine, the cancellable bag, consume, @ObservedDependency, OSLog/SwiftUI) included os(WASI). Switched the Combine/OSLog/SwiftUI ones to canImport(...). Keychain (SecureState) and iCloud KV (SyncState) stay Apple-only.

Verification

163 tests pass on macOS, against both published Cache 2.1.2 and the WASM/Linux-fixed Cache branch.

Sequencing

Full WebAssembly build and Linux collection-typed state also need the Cache fix — 0xLeif/Cache#30. Recommend merging + releasing that first, then bumping AppState's Cache pin. #150 observation delivery is AppState-only and verified by this PR's Linux CI independently.

Addresses #149, #150, #151.

[gemini-code-assist]

Code Review

This pull request refactors platform-specific checks (#if !os(Linux) && !os(Windows)) to use feature-based checks (canImport) across the codebase, and adds a detailed note regarding a known Swift-on-Linux runtime crash (Issue #151). It also introduces notifyChange() calls to several state setters to ensure proper observation. However, a critical thread-safety issue was identified: because these state setters (such as StoredState, FileState, SyncState, and DependencySlice) are not restricted to @MainActor, they can be invoked from background threads. This would trigger a main-thread assertion in notifyChange() and crash the application. It is highly recommended to make notifyChange() thread-safe or dispatch it to the main thread.

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