Make dump reader caches safe for concurrent reads

[leculver]

Summary

Makes the file-backed dump reader cache layer safe for concurrent readers without changing DAC behavior.

This PR fixes managed cache publication in the dump-reader/cache-entry layer:

• publishes CoreDumpReader and ELF-derived lazy state with atomic/locked first-publisher patterns
• makes Windows cached reader pages immutable once published
• returns only ArrayPool buffers that lost publication and were never visible to readers
• avoids returning published evicted buffers to ArrayPool while in-flight readers might still hold references

Part of #420.

Validation

• Built as part of the split branch verification: dotnet build src/Microsoft.Diagnostics.Runtime/Microsoft.Diagnostics.Runtime.csproj --framework net10.0 -- passed with 0 warnings and 0 errors.
• This split branch was recombined with the other reduced branches and compared against lockfree-mmf-threadsafe; all non-stress paths matched the original combined branch.

[Copilot]

Pull request overview

This PR updates ClrMD’s file-backed dump reader caching and ELF/core-dump lazy state initialization to be safe under concurrent readers, primarily by switching from per-page locks to atomic “first publisher wins” patterns and ensuring published pages remain immutable/usable by in-flight readers.

Changes:

• Reworked Windows segment cache entry page population to use Volatile.Read + Interlocked.CompareExchange publication, with a hook to discard unpublished pages (for returning unused pooled buffers).
• Updated the ArrayPool-backed cache entry eviction behavior to avoid returning published/evicted buffers to ArrayPool while concurrent readers may still hold references.
• Converted several Linux ELF/core-dump lazily initialized caches to lock-free or locked-first-publisher patterns (Volatile/Interlocked or explicit lock + publication flag), and marked the Linux coredump reader as thread-safe.

Show a summary per file

File	Description
src/Microsoft.Diagnostics.Runtime/Windows/CacheEntryBase.cs	Removes per-page RW locks and publishes cache pages via CAS; adds DiscardUnusedPage for safe cleanup of unpublished pages.
src/Microsoft.Diagnostics.Runtime/Windows/ArrayPoolBasedCacheEntry.cs	Adapts page-in to possible concurrent execution; returns only unpublished loser buffers to ArrayPool, and makes eviction CAS-based without returning published buffers.
src/Microsoft.Diagnostics.Runtime/Linux/ElfFile.cs	Makes DynamicSection and virtual-address reader initialization safe for concurrent callers using Volatile/CAS.
src/Microsoft.Diagnostics.Runtime/Linux/ElfCoreFile.cs	Makes loaded-images publication lock-free, virtual address space init lock-free, and auxv table loading single-writer with a publication flag.
src/Microsoft.Diagnostics.Runtime/DataReaders/Core/CoreDumpReader.cs	Makes coredump reader caches publish safely under concurrency; changes IsThreadSafe to true and replaces nullable bool cache with atomic tri-state int.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 1