Add direct LLVM runtime lowering from ResolvedProgram

[neysofu]

Summary

• describe how the LLVM backend now compiles directly from ResolvedProgram, sharing runtime_layout helpers with the QBE path
• note verification still depends on the QBE pipeline and prove remains a runtime no-op in the LLVM path
• mention updates to agents docs and runtime documentation to reflect the new workflow

Testing

• Not run (not requested)