Validate GGUF tensor data bounds

[mikemikimike]

Summary

• Validate GGUF tensor data offsets and byte ranges against the model file before loading tensor data.
• Use the same bounds checks for mmap and non-mmap loading paths.
• Add a regression test covering valid loading and truncated or overflowing tensor offsets.

Fixes #283

Issue: #283

Tests

• cmake --build build --config Release -j2
• ctest --test-dir build --verbose --timeout 900 (19/19 passed)