Add analyzer rules for ConceptAs conventions (CRARCH0027, CRARCH0028)

[Copilot]

Adds two Roslyn analyzers enforcing ConceptAs domain value conventions with actionable diagnostic messages.

Rules

CRARCH0027: ConceptAs must have NotSet sentinel

• Detects records inheriting ConceptAs<T> without a static readonly sentinel field
• Accepts NotSet, Empty, Null, or None as valid names
• Diagnostic message includes type-specific examples (Guid.Empty, string.Empty, 0)

CRARCH0028: Guid-backed concepts must have New() factory

• Detects ConceptAs<Guid> records without a static New() method
• Only triggers for Guid-backed concepts, ignores string/int/etc
• Diagnostic message includes factory implementation example

CRARCH0029: Reserved

• Rule ID allocated, infrastructure in place but deferred
• Primitive type detection requires additional semantic analysis refinement

Example

Before:

public record AuthorId(Guid Value) : ConceptAs<Guid>(Value)
{
}
// ⚠️ CRARCH0027: Record 'AuthorId' must have NotSet sentinel
// ⚠️ CRARCH0028: Record 'AuthorId' must have New() factory

After:

public record AuthorId(Guid Value) : ConceptAs<Guid>(Value)
{
    public static readonly AuthorId NotSet = new(Guid.Empty);
    public static AuthorId New() => new(Guid.NewGuid());
}

Implementation

• ConceptAsMustHaveNotSetSentinelRule.cs - checks for sentinel field by name
• GuidConceptMustHaveNewFactoryRule.cs - checks Guid concepts for New() method
• Wired into NamedTypeAnalyzer symbol analysis pipeline
• 7 new BDD-style specs, all existing tests pass (108/108)