Add tg3_validate() glTF 2.0 validation API to tiny_gltf_v3

[Copilot]

tinygltf v3 had error codes and an error stack for validation but no post-parse validation function. This adds tg3_validate() — a standalone validator that checks a parsed tg3_model against the glTF 2.0 spec.

API

/* C */
tg3_error_code tg3_validate(const tg3_model *model, tg3_error_stack *errors);

/* C++ wrapper */
tg3_error_code tinygltf3::validate(const Model &model, ErrorStack &errors);

Typical usage: parse first, then validate independently (or validate after programmatic model construction).

tg3_model model;  tg3_error_stack errors;
tg3_error_stack_init(&errors);
tg3_parse_auto(&model, &errors, data, size, "", 0, &opts);
tg3_error_stack_free(&errors);
tg3_error_stack_init(&errors);

if (tg3_validate(&model, &errors) != TG3_OK) {
    for (uint32_t i = 0; i < tg3_errors_count(&errors); i++) {
        const tg3_error_entry *e = tg3_errors_get(&errors, i);
        fprintf(stderr, "%s: %s\n", e->json_path, e->message);
    }
}
tg3_model_free(&model);
tg3_error_stack_free(&errors);

Validation coverage

• Asset — version required, warns if not "2.0"
• Buffer views — buffer index, byteOffset+byteLength within buffer bounds, byteStride range [4,252] and 4-alignment (per spec §3.6.2.4)
• Accessors — valid componentType/type, count ≥ 1, byte bounds and alignment within buffer view, sparse sub-structure refs
• Meshes — at least one primitive per mesh, attribute/index/material accessor refs, index accessor must be SCALAR+unsigned, mode in [0,6]
• Nodes — camera/skin/mesh/light refs, children refs, no self-reference
• Skins — joints non-empty, all joint indices valid, IBM accessor must be MAT4 FLOAT with count ≥ joints
• Animations — sampler input/output accessor refs, interpolation values, channel target path one of translation|rotation|scale|weights
• Textures — sampler and source image indices
• Materials — alphaMode enum, all texture info indices
• Cameras — type required, perspective yfov/znear > 0, orthographic xmag/ymag ≠ 0, znear ≥ 0, zfar > znear
• Scenes — node indices
• Images — bufferView index, warns on missing URI/bufferView/data

Error message lifetime fix

tg3_error_stack gains an internal msg_arena_ field. tg3_validate() allocates both message and json_path strings into this arena so they remain valid after the function returns. tg3_error_stack_free() destroys it. No change to existing parser behaviour.

Tests

New tests/v3/tester_v3.cc (22 test cases, 53 assertions) and corresponding tester_v3 CMake target.

Original prompt

Add glTF validation feature. for tinygltf v3 only

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