Test flake in TestMonitorAsyncWaitForWithNestedWaitUntil

```
=== RUN TestMonitorAsyncWaitForWithNestedWaitUntil
2026-07-28T14:54:17.081Z DEBUG simplex/monitor_test.go:132 Executing f {"taskID": 0}
2026-07-28T14:54:17.082Z DEBUG simplex/monitor_test.go:132 Scheduling task {"timeout": "10ms", "deadline": "2026-07-28T14:54:17.091Z"}
2026-07-28T14:54:17.082Z DEBUG simplex/monitor_test.go:132 Task executed {"taskID": 0}
2026-07-28T14:54:17.082Z DEBUG simplex/monitor_test.go:132 Ticked {"taskID": 1, "time": "2026-07-28T14:54:17.091Z"}
panic: test timed out after 10m0s
running tests:
TestMonitorAsyncWaitForWithNestedWaitUntil (10m0s)

goroutine 35 [running]:
testing.(*M).startAlarm.func1()
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/testing/testing.go:2682 +0x605
created by time.goFunc
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/time/sleep.go:215 +0x45

goroutine 1 [chan receive]:
testing.(*T).Run(0xc000102fc0, {0xaabc8c, 0x2a}, 0xabb248)
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/testing/testing.go:2005 +0x9fe
testing.runTests.func1(0xc000102fc0)
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/testing/testing.go:2477 +0x86
testing.tRunner(0xc000102fc0, 0xc0001779d8)
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/testing/testing.go:1934 +0x21d
testing.runTests(0xc00012a1b0, {0xe2d980, 0x73, 0x73}, {0xc00013ad28?, 0xc000177a98?, 0xe31ca0?})
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/testing/testing.go:2475 +0x96d
testing.(*M).Run(0xc00011ea00)
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/testing/testing.go:2337 +0xed5
go.uber.org/goleak.VerifyTestMain({0xb4ffe0, 0xc00011ea00}, {0x0, 0x0, 0x0})
/home/runner/go/pkg/mod/go.uber.org/goleak@v1.3.0/testmain.go:53 +0x65
github.com/ava-labs/simplex/simplex.TestMain(...)
/home/runner/work/Simplex/Simplex/simplex/leak_test.go:13
main.main()
_testmain.go:279 +0x185

goroutine 32 [sync.WaitGroup.Wait]:
sync.runtime_SemacquireWaitGroup(0xc00011b150?, 0x0?)
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/runtime/sema.go:114 +0x2e
sync.(*WaitGroup).Wait(0xc00011b150)
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/sync/waitgroup.go:206 +0xd3
github.com/ava-labs/simplex/simplex.TestMonitorAsyncWaitForWithNestedWaitUntil(0xc00022e380)
/home/runner/work/Simplex/Simplex/simplex/monitor_test.go:120 +0x212
testing.tRunner(0xc00022e380, 0xabb248)
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/testing/testing.go:1934 +0x21d
created by testing.(*T).Run in goroutine 1
/home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64/src/testing/testing.go:1997 +0x9d3

goroutine 33 [select]:
github.com/ava-labs/simplex/simplex.(*Monitor).run(0xc00012c4e0)
/home/runner/work/Simplex/Simplex/simplex/monitor.go:81 +0x22a
created by github.com/ava-labs/simplex/simplex.NewMonitor in goroutine 32
/home/runner/work/Simplex/Simplex/simplex/monitor.go:43 +0x2ac
FAIL github.com/ava-labs/simplex/simplex 600.106s
```

Start by running TestMonitorAsyncWaitForWithNestedWaitUntil in simplex/monitor_test.go and inspect the test around line 120. Trace the monitor behavior through simplex/monitor.go:81, using the reported timeout and goroutine dump to identify why the nested wait can stall. Done means the test completes reliably without the 10-minute timeout.

Written by the indexing model from the issue text.