C#: Preserve MinimumViableSpacing fixes when Roslyn formatting is a no-op

[knutwannheden]

Motivation

The MakeFieldReadOnly recipe in recipes-csharp produces readonlyList<T> instead of readonly List<T> when adding the readonly modifier to fields. The readonly keyword is missing a trailing space before the type name.

The root cause is in RoslynFormatter: when MinimumViableSpacingVisitor (MVS) correctly adds a space between the modifier and the type expression, but Roslyn formatting makes no additional changes, the formatter short-circuits at source == formattedSource and returns the original CU — discarding the MVS fix.

The flow was:

1. MVS adds space to type expression prefix → mvsCu (correct: readonly List<int>)
2. Print mvsCu → correct output
3. Roslyn formats → no change (already correct)
4. source == formattedSource → returns originalCu (no space!) → readonlyList<int>

Summary

• When Roslyn is a no-op but MVS made changes, reconcile the MVS changes within the target subtrees instead of discarding them
• Fixed in both Format() (single subtree) and FormatSpans() (multi-target batch)
• Added integration tests simulating MakeFieldReadOnly recipe behavior (adding modifier + MaybeAutoFormat)
• Added MVS unit tests confirming the visitor correctly handles added modifiers

Test plan

• New AutoFormatAfterAddingReadonlyModifierToGenericField — adds readonly to List<int> field via visitor + MaybeAutoFormat, verifies readonly List<int> output
• New AutoFormatAfterAddingReadonlyModifierToSimpleField — same for int field
• New MVS unit tests (AddedModifierToFieldWithGenericType, AddedModifierToFieldWithSimpleType)
• Full C# test suite (1771 tests pass)