fix: match tree order to unity hierarchy

[hashiiiii]

Summary

Order the diff tree the way Unity's Hierarchy window does: siblings follow the parent Transform's m_Children array, and scene roots follow SceneRoots.m_Roots (or m_RootOrder for old scene formats) instead of YAML document order.

Motivation

Unity serializes YAML documents in an order unrelated (often exactly reversed) to the Hierarchy. core/src/tree.zig built the tree in document order, so the editor window, CLI, and extension all showed objects in the wrong order — e.g. the Playground scene fixture rendered Floor → Crate → Sun while the Hierarchy shows Sun → Crate → Floor. One fix in core corrects all three surfaces.

Closes #150

Changes

• Reorder each sibling list to the parent Transform/RectTransform m_Children order; PrefabInstance children rank via their stripped root Transform (reusing ownerNodeIdOfTransform).
• Reorder roots by SceneRoots.m_Roots (class 1660057539, after side preferred) when present, else stable-sort by m_RootOrder (PrefabInstance roots read it from m_Modifications); prefabs and .asset files fall through untouched.
• Nodes absent from the after-side ordering data (removed objects) keep insertion order after the ranked ones.
• Extract modificationValue from instanceName for reuse.
• Add five ordering tests (m_Children siblings, SceneRoots roots, m_RootOrder fallback, instance-among-siblings, removed-child placement).

Testing

• zig build test --summary all — 168/168 tests passed (the five new tests failed before the fix with reversed order, e.g. expected 2, found 3).
• pnpm test (extension) — 187/187 passed.
• dotnet test (editor DotNetTests~) — 34/34 passed.
• CLI on real fixtures:
  • prefablens site/fixtures/{before,after}/Assets/Scenes/Playground.unity now prints roots Sun → Crate → Floor, matching SceneRoots.m_Roots (previously Floor → Crate → Sun).
  • Empty-vs-Robot.prefab diff now prints Robot → Arm → Sensor, matching the root Transform's m_Children (document order is Sensor → Arm).