fix(adapter): Add missing diagnostics, validation, and deterministic ordering

[Copilot]

Addresses feedback from PR #108 review thread. Adds 5 new diagnostics (PKADP014-PKADP017), strengthens validation, ensures deterministic output, and documents known limitation with multiple adapters sharing the same adaptee.

New Diagnostics

• PKADP014: Reject nested or generic host classes
• PKADP015: Require mapping methods be public or internal (accessible from generated adapter)
• PKADP016: Reject static members on target contracts
• PKADP017: Reject ref return and ref readonly return members

Validation Enhancements

• Host classes must now be top-level and non-generic
• Mapping methods validated for accessibility (previously only checked static)
• Static members filtered from target contracts with diagnostic
• Ref-return methods/properties detected and rejected

Deterministic Ordering

Member emission now uses stable sort (Kind → Name → Fully-qualified signature) instead of traversal order, ensuring consistent output across compilations.

Documentation

• Updated diagnostics table with PKADP009-PKADP017
• Added Limitations section documenting mapping ambiguity when multiple [GenerateAdapter] attributes share the same adaptee type:

// ⚠️ Problematic: Ambiguous mappings
[GenerateAdapter(Target = typeof(IServiceA), Adaptee = typeof(Legacy))]
[GenerateAdapter(Target = typeof(IServiceB), Adaptee = typeof(Legacy))]
public static partial class Adapters
{
    // Both IServiceA and IServiceB have DoWork() - which target does this map to?
    [AdapterMap(TargetMember = nameof(IServiceA.DoWork))]
    public static void MapDoWork(Legacy adaptee) => ...
}

// ✅ Workaround: Separate hosts
[GenerateAdapter(Target = typeof(IServiceA), Adaptee = typeof(Legacy))]
public static partial class ServiceAAdapters { ... }

[GenerateAdapter(Target = typeof(IServiceB), Adaptee = typeof(Legacy))]
public static partial class ServiceBAdapters { ... }

Cleanup

• Removed unused using System.Collections.Immutable;
• Renamed test ErrorWhenTargetNotInterface → ErrorWhenTargetNotInterfaceOrAbstract

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