[AndreyStartsev]

Problem

In IfcProfile.Legacy, AssociateTypeObjectMaterials seeded the GlobalId of type-level IfcRelAssociatesMaterial entities from the STEP entity number of the relating IfcMaterial, IfcMaterialList or IfcMaterialLayerSet (type-materials:{relatingMaterial} / type-layer-set:{layerSet}). Adding or removing unrelated elements or properties earlier in the file shifted those STEP #N numbers and changed the GlobalId of every type-level material relation, even though product-level material relations (materials:{materialKey} and material-layers:{group.Key}) already used stable Revit material/layer keys.

Closes #88

Changes

• Track (int EntityId, string Seed) in relatingMaterialByProductEntity (materialKey, e.g. "0", "700", "710,720") and layerSetByProductEntity (ownType.TypeId or group.Key).
• Seed AssociateTypeObjectMaterials relation GlobalIds from type-materials:{materialSeed} and type-layer-set:{layerSetSeed} instead of STEP entity IDs.
• Update ComponentType_ReceivesTheMaterialOfItsFirstInstance (type-materials:0) and add LegacyTypeMaterialRelationGlobalIds_StayStableWhenEarlierStepEntitiesShift verifying single-material (IfcMaterial) and multi-material (IfcMaterialList) type-level relations stay identical when prepended elements/properties shift STEP entity IDs, while IfcProfile.Current keeps its existing MD5("{projectGlobalId}IfcRelAssociatesMaterial:{firstRelatedGlobalId}") seed.
• Record the intentional one-time GlobalId change for legacy type-level material relations under ### Breaking changes in CHANGELOG.md.

Export comparison

• IfcProfile.Current (default) and JSON exports: byte-for-byte unchanged (ApplyCurrentMaterialRelationGlobalIds rewrites material relation GlobalIds in Current profile, and JSON exports do not touch IfcExporter).
• IfcProfile.Legacy: product-level IfcRelAssociatesMaterial entities (materials:{materialKey}, material-layers:{group.Key}) are unchanged; type-level IfcRelAssociatesMaterial entities change their GlobalId once from IfcGuid.FromName("type-materials:<stepEntityId>") to IfcGuid.FromName("type-materials:<materialElementIds>") and remain stable across unrelated STEP entity shifts.

Verification

• dotnet build RvtIO.slnx -c Release — 0 errors, 0 warnings in src/.
• dotnet test tests/RvtIO.Tests/RvtIO.Tests.csproj -c Release --no-build --filter "FullyQualifiedName~IfcExporterTypeObjectsTests|FullyQualifiedName~HostMaterialUsageTests|FullyQualifiedName~HostTypeLayerSetGroupingTests|FullyQualifiedName~IfcMepMaterialsTests" — 298 passed, 0 failed (net8.0 and net10.0). Private-fixture [SkippableFact] tests outside this filter are skipped when RVTIO_TEST_FILES is unset.
• dotnet docfx docfx.json --warningsAsErrors — 0 warnings, 0 errors.