[cDAC] Cache resolved read-only metadata address per module in EcmaMetadata_1

[max-charlamb]

Fixes part of #128717.

GetReadOnlyMetadataAddress walked the PE headers on every call and asked PEReader to also PrefetchMetadata, which eagerly read the entire metadata blob into memory and then discarded it (we only ever read PEHeaders.MetadataStartOffset / MetadataSize from the PEReader before disposing it).

On dumps where the metadata pages are not all captured, the prefetch threw VirtualReadException before the method could cache its result, so every subsequent call repeated the full PE walk and threw again. The exceptions were swallowed by callers' fallbacks (e.g. SOSDacImpl.GetMethodTableName -> DacStreams.StringFromEEAddress), so functionality was unaffected but performance suffered badly.

This PR:

• Drops PrefetchMetadata (we never use the prefetched bytes; callers read the metadata span themselves via target.ReadBuffer).
• Adds a small per-handle cache that Flush() clears alongside _metadata.
• Promotes ModuleHandle to a record struct so dictionary lookups use compiler-generated IEquatable<T>/GetHashCode instead of reflection-based ValueType.Equals (~6x faster in a microbenchmark).

Measurements

Measured on SOS.WebApp3.Heap.dmp via a local benchmark harness (GetMethodTableData + GetMethodTableName + GetMethodTableFieldData + GetObjectData over the full GC heap, 3 iterations):

Metric	Before	After	Delta
Cold-iter reads	137,522	61,306	-55%
Warm-iter reads	99,198	19,518	-80%
Total reads	385,928	150,351	-61%
Failed reads	1,992	1,992	unchanged
Name resolution ok/err	686/0	686/0	unchanged

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[Copilot]

Pull request overview

Caches the resolved read-only metadata TargetSpan per ModuleHandle in EcmaMetadata_1 and drops the unused PrefetchMetadata flag from PEReader, eliminating repeated PE walks and per-call eager metadata reads that were throwing VirtualReadException on dumps with partial metadata pages. ModuleHandle is promoted to a record struct to give dictionary lookups compiler-generated value equality instead of reflection-based ValueType.Equals.

Changes:

• Add _readOnlyMetadataAddress cache in EcmaMetadata_1, cleared in Flush() alongside _metadata.
• Stop requesting PEStreamOptions.PrefetchMetadata since callers read metadata bytes themselves via target.ReadBuffer.
• Convert ModuleHandle from a hand-written readonly struct to a readonly record struct for fast value equality/hashing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/EcmaMetadata_1.cs	Adds per-module cache for the read-only metadata address and removes PrefetchMetadata.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/Contracts/ILoader.cs	Converts ModuleHandle to a readonly record struct for compiler-generated equality.