Wrap GLTF material inventory uploads in binary LLSD

[lyrae-versebound]

Summary

• encode the second-stage agent and task material inventory uploads as binary LLSD
• carry version, type, and the material JSON in the envelope data field
• preserve the existing two-stage capability flow
• add deterministic request-body coverage for the actual uploaded asset

Motivation

LibreMetaverse currently uploads raw GLTF JSON during the second stage, but the capability verifier expects the viewer-compatible binary LLSD material envelope. Wrapping the JSON as version, type, and data matches the reference viewer behavior and allows the upload to complete.

Validation

• exact binary-LLSD regression passed on net8.0, net9.0, and net10.0
• complete GLTFMaterialUploadTests fixture passed 4/4 on net481, net8.0, net9.0, and net10.0 after the cancellation-token compatibility fix
• net9.0 material/BinarySD neighboring group passed 51/51
• net9.0 inventory/upload neighboring group passed 25/25
• affected net9.0 Release build passed with 0 warnings and 0 errors
• previously completed full net10.0 candidate and untouched-base controls each passed 1211 tests with 19 skipped and 0 failed
• repository CI passed its full cross-platform and multi-framework matrix on the original candidate
• git diff --check passed

A separate full net9.0 suite was not run locally; local net9.0 validation was bounded to the affected fixture, exact regression, neighboring groups, and affected library build. Repository CI runs the complete cross-platform test matrix.

Breaking changes

None. This PR does not remove or rename any public API.

Limitations

• no live-grid upload was performed
• the compact LibreMetaverse binary header versus viewer spelling was not independently tested against a live server
• a possible viewer-buffer trailing NUL was not independently tested

Related issues

Fixes #172