Add LOD cross-fade support, null safety, and consistent LODGroup initialization

[Copilot]

AutoLOD creates LODGroup components across three code paths but never configures fade mode, uses inconsistent initialization patterns, and lacks null guards on renderer arrays.

LOD fade mode

• New LODFadeMode and animateCrossFading settings in AutoLODSettingsData, LODImportSettings, and the Preferences UI
• Applied to all three LODGroup creation sites: AutoLOD.GenerateLODs, ModelImporterLODGenerator.CreateLODGroup, LODVolume_Editor.GenerateHLOD/GenerateLODs
• Per-model override via LODDataEditor inspector syncs fade settings

Consistent LODGroup initialization

ModelImporterLODGenerator.CreateLODGroup now uses the ForceLOD(0) / SetLODs / RecalculateBounds / ForceLOD(-1) pattern already used in AutoLOD.cs, ensuring deterministic initial LOD state.

Null safety

• ModelImporterLODGenerator.CreateLODGroup: filters null renderers from LOD arrays before SetLODs
• Extensions.HasLODChain: guards lod.renderers for null before .Length
• LODGroupExtensions.SetRenderersEnabled: early-returns on null lods, skips null renderers

LODGroupHelper.InvalidateCache()

New public method to flush all cached derived state (m_LODs, m_ReferencePoint, m_WorldSpaceSize, m_MaxLOD). The setter already does this on reassignment, but external callers modifying the underlying LODGroup directly had no way to reset stale cache.

// Configure fade mode on created LODGroups
lodGroup.fadeMode = autoLODSettingsData.FadeMode;
lodGroup.animateCrossFading = autoLODSettingsData.AnimateCrossFading;

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[Copilot]

Pull request overview

This PR adds configurable LOD cross-fade behavior across AutoLOD’s LODGroup creation paths, improves null-safety when working with renderer arrays, and introduces a cache invalidation API for LODGroupHelper to prevent stale derived data.

Changes:

• Introduces LODFadeMode and animateCrossFading settings in project preferences and import settings, and applies them when creating/configuring LODGroups.
• Adds null-safety around LOD renderer arrays (filtering null renderers before SetLODs, and guarding null arrays during iteration).
• Adds LODGroupHelper.InvalidateCache() to allow external callers to flush cached LODGroup-derived state.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Runtime/LODVolume_Editor.cs	Applies fade mode + cross-fade animation settings when generating scene/volume LODGroups.
Runtime/LODImportSettings.cs	Adds fade mode + animate cross-fading fields to per-asset import settings.
Runtime/LODGroupHelper.cs	Adds InvalidateCache() for flushing cached LODGroup-derived values.
Runtime/Extensions/LODGroupExtensions.cs	Adds null guards when enabling/disabling renderers via LOD arrays.
Runtime/AutoLODSettingsData.cs	Stores fade mode + cross-fade animation settings and exposes them via properties.
Editor/ModelImporterLODGenerator.cs	Filters null renderers, uses consistent ForceLOD init pattern, and applies fade settings on import-created LODGroups.
Editor/LODDataEditor.cs	Syncs fade settings into per-model import settings when toggling defaults/overrides.
Editor/Extensions.cs	Adds null guard for lod.renderers in HasLODChain().
Editor/AutoLODSettings.cs	Adds Preferences UI for selecting LOD fade mode and cross-fade animation.
Editor/AutoLOD.cs	Applies fade settings when generating LODGroups from the menu workflow.
CHANGELOG.md	Documents the new fade settings, cache invalidation, and null-safety fixes.

Comments suppressed due to low confidence (1)

Editor/LODDataEditor.cs:50

• LODImportSettings now contains fadeMode and animateCrossFading, but the custom LODImportSettingsDrawer currently does not render these fields. As a result, when Override Defaults is enabled, the per-model import settings UI won’t allow editing the new fade settings (and the PR description’s “per-model override” isn’t actually achievable). Update the drawer to display these properties (and ideally only show animateCrossFading when fade mode != None, matching Preferences).

                m_ImportSettings.FindPropertyRelative("fadeMode").enumValueIndex = (int)autoLODSettingsData.FadeMode;
                m_ImportSettings.FindPropertyRelative("animateCrossFading").boolValue = autoLODSettingsData.AnimateCrossFading;
            }

            if (settingsOverridden)
            {
                EditorGUILayout.PropertyField(m_ImportSettings, new GUIContent("Import Settings"), true);

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