// Primary workaround for #524 — combine custom-type series:
var sSum = s1.ZipInto(s2, (a, b) => a + b);               // inner join
var sProd = s1.ZipInto(s2, (a, b) => a * b);

// Get aligned (V1, V2) tuples for further LINQ processing:
var pairs = s1.ZipInner(s2);   // Series(K, (V1, V2))
var combined = pairs.Select(kvp => kvp.Value.Item1 + kvp.Value.Item2);

// Left join; produce missing when right value is absent:
var result = s1.ZipAlignInto(s2,
    (a, b) => b.HasValue
                  ? OptionalValue.Create(a.Value + b.Value)
                  : OptionalValue(MyType).Missing,
    JoinKind.Left,
    Lookup.Exact);
```

## Implementation

All three methods delegate to their existing F# counterparts
(`Series.zipInto`, `Series.zipInner`, `Series.zipAlignInto`).  No new
behaviour is introduced — this is purely a C# usability layer.

`ZipAlignInto` uses `OptionalValue(T)` (Deedle's own missing-value type)
rather than `FSharpOption(T)`, which is more natural from C#.

## Test Status

```
Passed!  - Failed: 0, Passed: 669, Skipped: 0, Total: 669