feat: unify runtime function scheduling

[V3RON]

Why this matters

Runtime functions and headless tasks represented the same product idea: run registered JavaScript on a named runtime without mounting UI. Keeping them as separate concepts made the API harder to teach, harder to type, and harder to mirror consistently across JS and native callers.

This PR unifies that model around runtime functions. A runtime function is now the single unit of background work, and the caller chooses whether it needs a result or only needs the work accepted/queued. That gives app authors one registration model, one Metro discovery path, one native queue, and one mental model for cross-runtime work.

New API structure

The JS API now has two primary invocation shapes:

await call(runtimeFunction).on(runtimeName)(...args);
await schedule(runtimeFunction).on(runtimeName)(...args);

• call(...) is awaitable and returns the runtime function result.
• schedule(...) is fire-and-forget from the function-result perspective. Its promise resolves when native accepts or queues the work.
• schedule(...) is typed to accept only runtime functions whose resolved return type is void.
• Metro keeps directive shortcuts as awaitable call semantics.
• Runtime function helpers expose runOn(...) for awaitable calls and scheduleOn(...) for scheduled calls.

Native callers now use the same vocabulary:

ThreadedRuntime.call(context, runtimeName, functionId, argsJson, promise)
ThreadedRuntime.schedule(context, runtimeName, functionId, argsJson)

C++/Objective-C++ scheduling uses:

nativecompose::threadedruntime::schedule(runtimeName, functionId, argsJson);

Summary of changes

• Removes the separate headless task registry and dispatch path.
• Adds schedule to the JS public API, native modules, Android/iOS runtime queues, C++ dispatcher, and Nitro runtime-function surface.
• Keeps call and schedule on a shared per-runtime queue so dispatch order is preserved.
• Updates Metro transformation so schedule(fn).on(runtime)(...) compiles to fn.scheduleOn(runtime, ...).
• Converts the two-runtime example from headless task registration to named scheduled runtime functions.
• Updates docs to describe background work through scheduled runtime functions instead of headless tasks.

Verification

• bunx tsc --noEmit -p example/tsconfig.json
• ./gradlew :app:compileDebugKotlin
• ./gradlew :react-native-runtimes_core:externalNativeBuildDebug
• In-memory TypeScript check confirms schedule(runtimeFunction(() => 1)) is rejected.
• bun test:harness --harnessPlatform=ios
• bun test:harness --harnessPlatform=android