fix(types): preserve async values through await inference

Problem

AwaitExpr currently falls through the checker's default Void result. As a consequence, locals initialized from await async_call() are typed void, and both examples/async_basic.home and examples/async.home fail when those locals are returned.

The checker also exposes an async call as its payload type directly, so it cannot reject await on ordinary values or distinguish a future from its resolved value.

Scope

• Represent async call results as Future<T> in the semantic type system.
• Mark function signatures as async without changing body return checking.
• Unwrap Future<T> at await and reject awaiting non-future values.
• Cover direct async calls, concrete awaited result types, and invalid awaits.
• Re-run the examples acceptance gate.

Acceptance criteria

• let value = await async_fn() infers the declared payload type.
• Calling an async function without awaiting produces Future<T>, not T.
• await 1 is a type error.
• async_basic.home and async.home pass home check.

Part of #779 and #814.

Completed by 32875fc0d on origin/main. Async function signatures now retain an is_async bit, calls synthesize Future<T>, and await is the checked boundary that unwraps T; un-awaited futures no longer masquerade as payload values and non-futures are rejected. Guarded verification: checker 72/72, no-JSC build at 2,819 MB peak, and examples improved 34/39 → 36/39 with both async_basic.home and async.home now passing.