MSL: fix robust struct zero initialization

[VladimirKras]

For struct loads, this changes the --msl-robust-buffer-access2 out-of-bounds fallback from Type(0) to Type{}. Metal rejects the former because MSL structs do not have a scalar constructor.

Scalar and vector construction remains unchanged. This also adds a regression shader with normal and optimized MSL references.

Testing:

• The new regression passes in normal and optimized modes using the repository's fixed glslang and SPIRV-Tools revisions.
• Existing scalar/vector robust-access output remains byte-identical from MSL 1.0 through 4.0; the struct form passes Metal runtime validation from MSL 1.1 through 4.0.
• The unpatched failure was reproduced, and the patched Vello 0.10 shell was visually verified end-to-end through guest Venus, MoltenVK, and Metal.

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