Validate legacy MSH headers before parsing

[CoffeeDrivenCoder]

Summary

• read the four legacy MSH header counts together and reject truncated headers
• reject negative counts before passing them to NumPy data reads
• report the actual expected texcoord count instead of the literal placeholder
• add regression coverage for empty, truncated, negative-count, and incomplete-texcoord files

Previously, empty or truncated files leaked an internal IndexError, while negative counts reached np.fromfile where -1 has special read-to-EOF semantics. These inputs now fail consistently with a descriptive ValueError.

Testing

• msh2obj_test.py: 4 tests passed, including the existing MSH-to-OBJ model equivalence test
• pyink --check on both changed Python files
• isort --check-only on both changed Python files
• git diff --check

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