[cDAC] Add EnC metadata and caching

[rcj1]

• Add capability for cDAC to read read-write metadata for Edit and Continue scenarios.
• Add caching schema for metadata
  • Under forward execution, metadata caches are not flushed. Metadata cache is updated upon changes to the metadata, as indicated by the metadata generation (for RefEmit) or by the EnC edit counter.
  • Under potentially backwards execution, metadata caches are flushed.
• Streamline docs

Fixes #129557

[Copilot]

Pull request overview

This PR expands the cDAC EcmaMetadata contract to support Edit-and-Continue (EnC) and read-write (RW) metadata scenarios by adding new VM data descriptors, walking RW metadata structures (MDInternalRW → CLiteWeightStgdbRW → CMiniMdRW), and caching metadata readers based on generation/edit counters.

Changes:

• Add new CoreCLR data descriptors and managed contract data models for RW metadata structures and EnC module state.
• Implement RW metadata reconstruction/serialization into a contiguous ECMA-335 metadata image, including heap/table coalescing.
• Adjust caching/flush behavior and update design documentation to reflect the new metadata sources and invalidation signals.

Show a summary per file

File	Description
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/EcmaMetadataUtils.cs	Adds helper to read metadata version string from a metadata root.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/DataType.cs	Adds new public DataType enum entries for RW metadata and EnC module types.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/StgPoolSeg.cs	New contract data model for StgPoolSeg.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/StgPool.cs	New contract data model for StgPool.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/PEAssembly.cs	Extends PEAssembly contract data to expose RW importer state/pointer.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/Module.cs	Adds MetadataGeneration for dynamic module saved-metadata invalidation.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/MDInternalRW.cs	New contract data model for MDInternalRW (RW metadata entrypoint).
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/EditAndContinueModule.cs	New contract data model for EnC edit counter (ApplyChangesCount).
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/CMiniMdSchema.cs	New contract data model for CMiniMd schema fields (heaps/sorted/row counts).
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/CMiniMdRW.cs	New contract data model for CMiniMdRW fields plus computed table segment addresses.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/CLiteWeightStgdbRW.cs	New contract data model for RW stgdb pointers and metadata image address.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/EcmaMetadata_1.cs	Implements RW metadata reading/serialization and generation-based caching.
src/coreclr/vm/peassembly.h	Exposes MDImport RW flag and MDImport pointer offsets to cDAC.
src/coreclr/vm/encee.h	Adds cDAC data exposure for EditAndContinueModule::m_applyChangesCount.
src/coreclr/vm/datadescriptor/datadescriptor.inc	Registers new cDAC types/fields for RW metadata and EnC module.
src/coreclr/vm/datadescriptor/datadescriptor.h	Adds metadata header includes needed for RW metadata descriptors.
src/coreclr/vm/datadescriptor/CMakeLists.txt	Adds compile definition enabling internal metadata API exposure for descriptor build.
src/coreclr/vm/ceeload.h	Adds Module::m_dwMetadataGeneration and exposes it via cDAC.
src/coreclr/vm/ceeload.cpp	Initializes/increments dynamic metadata generation counter.
src/coreclr/md/inc/stgpool.h	Adds cdac_data mappings for StgPool/StgPoolSeg fields.
src/coreclr/md/inc/metamodelrw.h	Adds m_fAll4ByteColumns and exposes RW heap/table offsets for cDAC.
src/coreclr/md/inc/metamodel.h	Adds cdac_data mapping for CMiniMdBase schema/tablecount fields.
src/coreclr/md/inc/liteweightstgdb.h	Adds cdac_data mapping for RW stgdb miniMd and metadata address.
src/coreclr/md/enc/metamodelrw.cpp	Initializes/sets m_fAll4ByteColumns under growth/expansion paths.
src/coreclr/md/enc/liteweightstgdbrw.cpp	Marks minimal-delta load as “all 4-byte columns” for RW model.
docs/design/datacontracts/EcmaMetadata.md	Updates the contract documentation and descriptor listing for RW metadata support.

Copilot's findings

Comments suppressed due to low confidence (1)

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/EcmaMetadata_1.cs:381

• GetReadWriteSavedMetadataAddress uses ProcessedData.GetOrAdd for Module and DynamicMetadata. Those structures can change when a dynamic module re-serializes its metadata; if GetMetadata is called again without an intervening Target.Flush, the cached descriptors can cause the updated metadata generation to be detected but the old address/size to still be used. Consider reading these descriptor types uncached for this mutable path.

    private TargetSpan GetReadWriteSavedMetadataAddress(ModuleHandle handle)
    {
        Data.Module module = target.ProcessedData.GetOrAdd<Data.Module>(handle.Address);
        Data.DynamicMetadata dynamicMetadata = target.ProcessedData.GetOrAdd<Data.DynamicMetadata>(module.DynamicMetadata);

        return new TargetSpan(dynamicMetadata.Data, dynamicMetadata.Size);

• Files reviewed: 26/26 changed files
• Comments generated: 8

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/EcmaMetadata_1.cs:380

• GetReadWriteSavedMetadataAddress uses ProcessedData.GetOrAdd for Module/DynamicMetadata. Dynamic modules can re-serialize and replace the DynamicMetadata buffer while the debuggee is paused (no Target.Flush), so cached IData snapshots can become stale even though GetMetadataGeneration detects a new generation. Use uncached IData reads here so the updated DynamicMetadata pointer/size is observed.

        Data.Module module = target.ProcessedData.GetOrAdd<Data.Module>(handle.Address);
        Data.DynamicMetadata dynamicMetadata = target.ProcessedData.GetOrAdd<Data.DynamicMetadata>(module.DynamicMetadata);

• Files reviewed: 26/26 changed files
• Comments generated: 6

[Copilot]

Copilot's findings

• Files reviewed: 29/29 changed files
• Comments generated: 5

[Copilot]

Copilot's findings

• Files reviewed: 30/30 changed files
• Comments generated: 5