Check a type once per write, so a nested expression body runs twice, not depth + 3 times [patch] - #205

Fixes #202

What was wrong

A property's ExpressionBodyFactory is rendered once to check it and once to write it. The check itself repeated at every level:

• ClassTemplate.WriteTo validated its whole subtree, and then each nested type's WriteTo validated its own subtree again.
• AddSourceFile validated every type, then AddClasses validated them again.

At depth 3 the callback ran 6 times. A stateful factory, such as a counter for unique names, wrote different code depending on how deeply its class was nested.

Change

A template write now opens a scope on the CodeBlocker (BeginTemplateWrite, internal). The scope records which ClassTemplates have been checked, by reference. ClassTemplate.WriteTo, ClassTemplate.Validate (for nested types), AddClasses and AddSourceFile skip a type that was already checked in the scope. The set is cleared when the outermost write ends, whether or not it succeeded.

• The documented "check, then write" pass stays: the callback runs exactly 2 times, however deeply its type is nested.
• Types are matched by reference, with a small comparer, because ReferenceEqualityComparer doesn't exist on netstandard2.0/2.1. A different type that a BodyFactory writes into the same CodeBlocker is still checked.

Tests

• New: ANestedPropertysExpressionBodyRunsOnceToCheckAndOnceToWrite. Depth 3 through AddSourceFile, ClassTemplate.WriteTo and AddClasses. Each expects 2 calls. Before the fix: 6.
• New: AStatefulExpressionBodyWritesTheSameCodeWhereverItsClassIsNested. This is the issue's repro: standalone and nested-in-a-file now both write First => 2 / Second => 3. Before, the nested case wrote 8 / 9.
• New guard: AClassWrittenFromInsideABodyFactoryIsStillChecked. An invalid type written from a method body callback still throws. It passes before and after, and keeps the skip from widening.
• With the fix reverted, the first two fail. Full suite on Linux / .NET 10: 380/380 passed.

This doesn't touch #176 (throwing BodyFactories leave half-written members). That needs the larger pre-render restructure.

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