[schloerke]

Fixes #33. An async function that ended with a loop or an if without else returned the exhaustion sentinel instead of a promise, so promises::then(async(function() while (FALSE) NULL)(), ~ NULL) failed. With an await() before the loop, the promise resolved to the sentinel instead of NULL. The cause was the fall-through return at the end of the state machine in walk_states(), which always returned exhausted(). A new fallthrough_value() helper now returns as_promise(NULL) for async functions and as_promise(exhausted()) for async generators.

Async generators had a second gap: calling one again after it was done returned the raw sentinel, while the docs say it always returns a promise. The exhausted check in the generator instance now wraps the sentinel in a promise for async generators, except on close = TRUE, which is only used for cleanup. Plain generators are unchanged.

Tests were written first and failed before the fix. The parser and async snapshots change only the final fall-through line. devtools::test() passes locally (317 pass, 2 skipped for reticulate).