Import glTF 2.0 files (.glb, .gltf) as mesh bodies (#1512)

glTF files open, insert and drop like 3MF/STL/OBJ: a component named after the file, the node tree as nested rigid components, repeated meshes as occurrences, metres to mm, Y up turned to Z up by default (up: z keeps axes), materials as PBR appearances. Bounded, hostile-input tested reader; no new persisted data. Closes #1505.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -53,6 +53,9 @@ rayon = "1.12"
 # packages (3MF)
 zip = { version = "3", default-features = false, features = ["deflate"] }
 quick-xml = "0.41"
+# glTF 2.0 import (MIT OR Apache-2.0): the document model and GLB container only; buffers are
+# read by crates/io itself, so no image decoding or file access comes with it.
+gltf = { version = "1.4", default-features = false, features = ["names"] }
 
 # fonts (sketch text outlines; MIT OR Apache-2.0, pure Rust)
 ttf-parser = { version = "0.25", default-features = false, features = ["std"] }