| Some value, _ -> formatPlainString <| value.ToString()

let toStringCache = System.Collections.Generic.Dictionary(obj, string)()
...
| Some value, _ ->
    match toStringCache.TryGetValue(value) with
    | true, s -> s
    | false, _ ->
        let s = formatPlainString <| value.ToString()
        if toStringCache.Count < 4096 then toStringCache.[value] <- s
        s
```

This works correctly for F# DUs because the compiler generates structural `Equals`/`GetHashCode` implementations, so the cache correctly groups equal DU values.

## Performance Impact

Per the issue reporter (#564), a 50k×100 frame with a two-valued DU column took **>80 seconds** to export. With a user-supplied `ToString()` override it dropped to ~6s. This fix achieves the same effect automatically — no user code changes needed.

## Trade-offs

- The cache is per-export (allocated inside `writeCsv`), so no global state.
- For high-cardinality reference types that don't override `Equals`/`GetHashCode`, the cache uses reference equality, which still avoids duplicate `ToString()` calls for the same object instance.
- The 4096-entry cap bounds memory to roughly ~100KB worst-case for long string values.

## Also included (Task 9 — Testing Improvements)

12 new tests for previously-untested Frame module functions:
- `Frame.mapColValues`, `Frame.mapCols`, `Frame.mapColKeys`
- `Frame.mapRowValues`, `Frame.mapRows`, `Frame.mapRowKeys`
- `Frame.filterColValues`, `Frame.filterCols`
- `SaveCsv` with discriminated union columns (regression test for #564)

## Test Status

All 505 tests pass on Ubuntu (`dotnet test tests/Deedle.Tests/Deedle.Tests.fsproj -c Release`):

```
Passed!  - Failed: 0, Passed: 505, Skipped: 0, Total: 505