Span.Progress returns Count > Total when a running child has Total == 0 (follow-up to #1208)

**Gorse version**

`master` (HEAD `b7e6aa9`, 2026-04-16). The relevant code lives at `common/monitor/progress.go:138-169`, introduced in #1208.

**Describe the bug**

After #1208 removed the divide-by-zero panic, the "zero-total child is complete" fallback can push `parentCount` above `parentTotal` when:

- the parent has already consumed its own `count` (`s.count == s.total`), and
- a running child is reporting `Total == 0`.

With a parent `total=2, count=2` and one running zero-total child:

```
childTotal (max, fallback) = 1
parentTotal = s.total * childTotal = 2
parentCount = s.count * childTotal = 2
+ childTotal (zero-total child branch) = 3
-> Progress{Count: 3, Total: 2} // 150%
```

The dashboard progress bar (`/api/dashboard/tasks`) consumes this value directly and renders `>100%`.

**To Reproduce**

```go
// common/monitor/progress_overflow_test.go
package monitor

import (
"context"
"testing"
)

func TestProgress_ZeroTotalChildExceeds100Percent(t *testing.T) {
_, parent := Start(context.Background(), "parent", 2)
parent.Add(2)

ctx := context.WithValue(context.Background(), spanKeyName, parent)
_, _ = Start(ctx, "child", 0)

got := parent.Progress()
t.Logf("parent.Progress() -> Count=%d Total=%d", got.Count, got.Total)
if got.Count > got.Total {
t.Fatalf("Count (%d) exceeds Total (%d)", got.Count, got.Total)
}
}
```

`go test -run TestProgress_ZeroTotal -v ./common/monitor/...` fails on HEAD with `Count (3) exceeds Total (2)`.

**Expected behavior**

`Progress.Count <= Progress.Total` for every combination of parent/child totals.

**Additional context / suggested fix**

A one-line clamp before returning:

```go
if parentCount > parentTotal {
parentCount = parentTotal
}
return Progress{..., Count: parentCount, Total: parentTotal, ...}
```

PR with the clamp + regression test is being opened against this issue.