CPU topology v2 phase 1: introduce CpuSelection and regression tests

[PrimeBuild-pc]

Description

• Introduce CpuSelection, ProcessorRef, CpuSelectionMetadata, CpuTopologySignature and minimal CpuTopologySnapshot.
• Adds regression tests for legacy long affinity mask bugs above 64 logical processors.
• Documents CoreMask.ToProcessorAffinity aliasing bug for CPU index 64.
• Adds conversion helpers from legacy affinity mask to CpuSelection and back when safely representable.
• Does not change runtime affinity application, UI, presets, profiles, or ProcessService behavior.

Checklist

• dotnet test "ThreadPilot_1.sln" --configuration Release --no-restore passes.
• No runtime behavior changes.
• Existing profile compatibility preserved.
• Legacy conversion returns null when selection cannot be represented as a safe single-group <64-bit mask.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 93.15068% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
Models/CpuSelection.cs	93.15%	7 Missing and 3 partials ⚠️

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