fix(async): preserve cancellation and add revision-aware job gate

packages/async/src/async.zigmodified+4-0

Changes to packages/async/src/async.zig

		⋯15 linesShow the 15 lines above this hunk@@ -16,6 +16,7 @@ pub const RawTask = task.RawTask;
16	16	pub const Runtime = runtime.Runtime;
17	17
18	18	test {
	19	+ _ = ;
19	20	_ = future;
20	21	_ = task;
21	22	_ = runtime;
		⋯1 lineShow the 1 line above this hunk@@ -23,3 +24,6 @@ test {
23	24	_ = work_stealing_deque;
24	25	_ = parker;
25	26	}
	27	+
	28	+/// Revision-aware event job gate; caller owns synchronization.
	29	+pub const CompletionGate = .Gate;