Refactor the internals of internal/tdp/vm to make it easy to add specialized decoding functions

[mcy]

This PR refactors much of hyperpb's internals to make it easy to experiment with alternative implementations for varint and UTF-8 decoding. Specifically, the tdp/vm package now has a number of internal packages that decouple core VM operations, the VM's interpreter, and decoding primitives.

This also includes the following improvements:

• Requires Go 1.26 and builds with GOEXPERIMENT=simd, to make simd/archsimd usable.
• Changes external and internal tools to be managed by go tool.
• Improves upload speed of remote tests in hypertest.
• hyperstencil can now handle //go:build constraints.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	⏩ skipped	✅ passed	✅ passed	Mar 12, 2026, 9:15 PM