Add Fragmentation Blame view for GC dump analysis

[Copilot]

Adds a view to identify objects preventing heap compaction by correlating Free objects (memory gaps) with their predecessors.

Implementation

Core Algorithm (FragmentationBlameStackSource.cs)

• Sorts heap objects by address O(n log n)
• Maps each Free object to its immediate predecessor in memory
• Aggregates Free object sizes as "fragmentation cost" per blamed object
• Delegates path-to-root queries to MemoryGraphStackSource for consistency

UI Integration (PerfViewData.cs)

• Adds view under Advanced Group for .gcDump files
• Configures CallTree tab as default (matches Generation views)
• Displays diagnostic stats: Free object count, total fragmentation MB

Usage

MyDump.gcDump
└── Advanced Group
    └── Fragmentation Blame  ← Shows blamed objects with paths to root

Metric column shows fragmentation cost (bytes), not object size. Helps identify:

• Pinned buffers preventing compaction
• Long-lived Gen2 objects fragmenting younger generations
• Static field references blocking collection

Edge Cases

• Consecutive Free objects: Only blames first real object, avoids double-counting
• No Free objects: Logs warning, returns empty view
• No predecessor: Logs warning for Free at heap start

Documentation

• User guide: documentation/FragmentationBlameView.md
• Conceptual tests: src/PerfView.Tests/FragmentationBlameStackSourceTests.cs

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