for await runs its body one iteration at a time (#498)

[ASDAlexander77]

The body of a for await loop ran as concurrent tasks. mlirGen(ForStatement) wrapped each iteration's body in an async.execute and added its token to an async.group created before the loop. It then waited for the whole group only once, with async.await_all after the loop. So the bodies ran together on the async runtime's threads:

• iterations interleaved (<7<8><9>);
• writes from one iteration were lost to another (keep.push, log += ...), under gc, rc and none;
• one run printed nothing at all.

Each iteration now awaits its body's token before going on, as await does since #440. The next value is requested only once the body is done, as in TypeScript. The group and the await_all are gone. The body stays an async.execute, because an await inside it needs one.

New test 00for_await_sequential.ts checks, over eight iterations, that the bodies' writes happen in order, every push is kept, and awaits inside the body run in order. On main it failed in all 6 ctest variants, three runs out of three. With the fix it passes every time, and under own too.

Gate	Result
Windows full suite (Release)	3840/3840
Linux (WSL) full suite	3825/3825
DefaultLib build + its own tests.ps1 -Model, release + debug, compile + JIT	gc 159/159; rc, none 158 passed + weakref_basic skipped (gc only); all "All tests passed"
own corpus	no flips, no first-error changes (461/619; the new test passes under own)

Not part of this change: break / continue in a for await body, and a nested for await, crash the compiler on main as well. Filed as #502.

Closes #498

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