Moves docs/inline-asm-design.md -> design/inline-asm-design.md (the
internal design record now lives under design/, separate from the
user-facing docs/). Updates all links: current/CHECKPOINT-ASM.md,
current/PLAN-ASM.md, current/PLAN-EXTERN-EXPORT.md (../docs -> ../design)
and docs/inline-assembly.md (same-dir -> ../design).
