[snkmcb]

Summary

• Replay host updates built a replacement tree for the loading-owner check, then built the same tree again through reload.
• Hand off the validated tree to the shared reload path. This removes one document validation, copy and indexing pass for each accepted host update.
• Cover a malformed replacement document in the host replay test, including the diagnostic path, the retained snapshot and a retry.

Compatibility

The public API, Replay v1/v2 encoding and the layout/paint commit boundary stay the same. Each replacement still has a fresh tree identity and expires earlier handles. A host update now allocates one tree identity for its candidate.

Verification

• Core Debug with examples off: all 64 contract tests passed, including Replay, host updates, serialization and offscreen capture.
• Hierarchy / Inspector Debug window-free smoke passed.
• Changed-file formatting, formatter self-check, documentation, release metadata and CI selection checks passed.

For the refactoring baseline, one clean local Core Debug run on 45e651f used Visual Studio 2026 x64, 2 build workers and 4 test workers. Configure took 2.10 s, build 106.33 s, and CTest 30.25 s. The recent hosted source-only PR spent 299 s in the Core contracts step. These are baseline observations.