Add UE5_8 engine version

[stevenlafl]

Add UE5_8 engine version

Toward #74: adds UE5_8 to EngineVersion so 5.8 titles can be targeted explicitly with --version UE5_8.

Note this does not make 5.8 support complete on its own: #72 documents a further 5.8 change (FFilePackageStoreEntry::ShaderMapHashes elements shrank from FSHAHash to 8 bytes, without an EIoContainerHeaderVersion bump) that affects titles whose store entries carry shader map hashes. The title used for verification below has none, so this PR is correct but orthogonal to that fix; with UE5_8 in the enum, a #72 fix gains a natural key for the write side.

Mapping

UE 5.8 introduced no new EIoStoreTocVersion, EIoContainerHeaderVersion, or EUnrealEngineObjectUE5Version entries (verified against the 5.7 and 5.8 release branches of the engine source), so UE5_8 maps to the same values as UE5_7:

• toc_version: ReplaceIoChunkHashWithIoHash
• container_header_version: SoftPackageReferencesOffset
• object_ue5_version: ImportTypeHierarchies

Verification

Tested against a commercial UE 5.8.1 title (updated from 5.7.4, so both cooks were available for comparison):

• retoc info on its shipped containers reports exactly the mapped versions above
• to-legacy --version UE5_8 extraction of its assets is byte-identical on the export payload to UE5_7-mode extraction
• to-zen --version UE5_8 round-trips extracted assets into correctly versioned containers, and containers built this way mount and load in the title's 5.8.1 runtime

Also adds UE5_8 cases to test_zen_asset_identity_conversion, mirroring the existing UE5_7 pattern (reusing the 5.6 fixtures, identical IoStore serialization).

Note: test_zen_asset_identity_conversion currently fails on master before reaching the 5.7/5.8 cases (failed to fill whole buffer on the UE5.4 fixtures; that directory has no AssetRegistry.bin and BP_Table_Lamp has no .metadata.json). The failure is identical with and without this change.

Disclosure

Developed in a Claude Code session: the AI assistant performed the engine-source version analysis and authored the change; I (the human) provided licensed UE source access, ran the real-game verification, and reviewed the result.