C#: a member or local named field inside a property accessor binds to C# 14's field keyword, so the getter silently returns the synthesized backing field (0) instead of the member, and a local named field fails with CS9273 #461

What's wrong

CSharpGenerator.EscapeIdentifier only escapes reserved keywords, and explicitly leaves contextual keywords alone on the grounds that they "are legal names everywhere else" (CSharpGenerator.cs L213-L240, from #441). field is the exception: since C# 14 (the default for net10.0), field inside a property accessor is the keyword for a synthesized backing field. So a reference to a member named field from a getter or setter body changes meaning without an error.

Repro (generated with CSharpGenerator, then built and run with the .NET 10 SDK)

The AST is a class Box with private int field = 42;, plus a property Field whose getter body is return field; and whose setter body is field = value;. There is also a property Twice whose getter declares a local int field = 2; return field;. The generator writes all of these verbatim:

public class Box
{
    private int field = 42;
    public int Field
    {
        get { return field; }
        set { field = value; }
    }
    public int Twice
    {
        get { int field = 2; return field; }
    }
}

• new Box().Field prints 0, not 42. The compiler gives only warning CS9258 ("the 'field' keyword binds to a synthesized backing field ... use 'this.field' or '@field' instead"). The setter likewise writes to the hidden backing field, and the declared member is never read (CS0414).
• Twice fails to compile: error CS9273 "'field' is a keyword within a property accessor. Rename the variable or use the identifier '@field' instead."

Every other target reads the declared member, so the C# output computes a different value from the same AST. The test suite would not catch this, because it is a warning and not a build failure.

Suggested fix / acceptance criteria

• Write an identifier named field as @field in C# at both the declaration and the reference. The simplest version does this everywhere, since @field is the same identifier wherever it appears. A narrower version does it only inside property accessor bodies. Either way, the escape goes through the existing EscapeIdentifier/EscapeReference hook so declarations and uses agree.
• Update the remark on ReservedWords so it no longer says every contextual keyword is safe.
• Add a test in CSharpGeneratedSourceCompilesTests covering the class above. It should build with no CS9258/CS9273, and running it should show the getter returning the member's value (42) and the setter updating that member.

Related: #106 (reserved-word escaping in general) and #441 (C# escaping, which deliberately excluded contextual keywords).