docs: add LLM.md for VM architecture

[Pattermesh]

Summary

• Adds an LLM.md discovery doc summarising the VM architecture, pluggable VM pattern, and thresholdvm M-Chain/F-Chain design context.
• Surfaces the CGO build-tag dispatch in evm/ / evm/cevm/ that PLUGGABLE.md mentions but does not enumerate file-by-file.
• Cross-refs README + PLUGGABLE for depth, and points to the existing nested evm/LLM.md and thresholdvm/LLM.md so the discovery surface is contiguous.

Test plan

• No code changes — docs only.
• Markdown renders correctly (preview-tested locally).
• All factual claims sourced from README.md, PLUGGABLE.md, Makefile, Dockerfile.plugin, and the existing evm/LLM.md / thresholdvm/LLM.md.
• @zatsch / maintainers review for accuracy on the VM-to-chain mapping and the build-tag table.