Add GEO_LOD vendor extension for per-part LOD

extensions/2.0/Vendor/GEO_LOD/README.md

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+# Bringing OpenFlight/GeoSTUDIO-Style, Per-Part LODs into glTF with GEO_LOD
+
+In visual-simulation toolchains (e.g. OpenFlight/Aechelon/Geo STUDIO), it�s routine to define per-part LOD variants directly in the model hierarchy. You might have a tank�s chassis split into high/med/low meshes under one grouping node, its turret under another, its wheels under yet another�all within a single file. No �root-only� swapping, and no need to carve your model into separate glTFs for each LOD.
+By contrast, glTF�s built-in LOD patterns (e.g. MSFT_lod) expect a single node that is the LOD switcher, listing mesh indices. That works, but it can�t express �here�s a chassis, with three mesh children,� or �here�s a turret behavior, with its own LOD children,� in one neat file.
+Enter GEO_LOD�a small vendor extension that you attach to any grouping node (having no mesh of its own) whose children (the entire sub-tree parented by the LOD node) are considered the LOD variant for that part. Your loader/runtime software then simply activates that LOD node (and its children) based on it passing the distance or screen-coverage switchin & switchout criteria. Any dynamic transformation applied above the LOD in the hierarchy will automatically be applied to the active LOD.
+
+![Here is a very simple (tank) example](images/tank-hierarchy.png)
+
+Each �Group� (in the above example) has no mesh itself. Holds three mesh-children for each LOD, and you can still animate or transform the group (rotate turret, deploy wheels, etc.) without rebinding when the LOD changes. 
+The following Geo STUDIO snapshot shows a more complex tank model as seen in Geo's tree view...
+
+![Here is a more complex (tank) example](images/t80_tree_view.png) 
+
+## Proposed JSON-Schema
+For the full machine-readable JSON Schema, see  
+[`schema.json`](schema.json)
+
+
+mode chooses your LOD metric
+switchinout = [ switchIn, switchOut ], with hysteresis built-in
+
+## Authoring Example (DISTANCE)
+
+![Authoring Example 1](images/authoring_example3.png)
+
+## Authoring Example (SCREEN_COVERAGE)
+
+![Authoring Example 2](images/authoring_example2.png)
+
+## Runtime pseudocode
+
+![Pseudocode](images/rendercode.png)
+
+## Why this works
+
+LOD nodes (with GEO_LOD) are pure grouping: no drawMesh call on them, they just gate traversal.
+Mesh nodes (without GEO_LOD) always render when reached.
+Nested LODs are supported naturally�if you have a behavior node above, you�ll traverse into it, hit its LOD child only when its thresholds match, and then render that child�s mesh subtree.
+Overlaps/blends happen automatically if two sibling LOD?groups both pass their tests�you could extend drawMesh to cross-fade in that case, or add extra logic in the traversal.
+This pattern exactly matches how GeoSTUDIO /OpenFlight engines embed LOD logic in grouping nodes and prune the tree as they walk it (building per-frame renderables).
+
+## Lastly
+
+I believe the introduction of this simple extension would hasten the adoption of glTF in the military training and simulation market -bringing with it all the inherent benefits of glTF/glb to that space.
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+Creator, and OpenFlight were developed by, and are registered trademarks of, Multigen-Paradigm (MPI) & currently maintained by Presagis Inc.

extensions/2.0/Vendor/GEO_LOD/schema.json

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+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "GEO_LOD",
+  "description": "Attach to any node whose subtree is a LOD variant—show it only when camera metric ∈ [switchIn, switchOut).",
+  "type": "object",
+
+  "not": { "required": [ "mesh" ] },
+
+  "properties": {
+    "mode": {
+      "type": "string",
+      "enum": [ "DISTANCE", "SCREEN_COVERAGE" ],
+      "description": "Which metric drives the switch: world-space distance or on-screen coverage."
+    },
+    "switchinout": {
+      "type": "array",
+      "items": { "type": "number" },
+      "minItems": 2,
+      "maxItems": 2,
+      "description": "[switchIn, switchOut] thresholds for this LOD node. "
+        + "For DISTANCE: switchIn = activation distance when approaching, "
+        + "switchOut = deactivation when receding. "
+        + "For SCREEN_COVERAGE: switchIn = activation coverage when coming into view, "
+        + "switchOut = deactivation when shrinking out of view."
+    }
+  },
+
+  "required": [ "mode", "switchinout" ],
+  "additionalProperties": false
+}