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).
Adds docs/inline-assembly.md — a how-to guide for inline assembly in the
docs/error-handling.md style: mental model, operands (inputs / value
outputs / naming + auto-naming rule), the result-type table, volatile,
clobbers, all three `-> @place` forms (write-through / read-write /
indirect-memory), multi-instruction `#string` templates, global asm +
lib-less extern, the JIT/AOT-yes vs `#run`-no execution model, a
cookbook (read-register, x86_64 syscall, divmod), and rules of thumb.
All aarch64 snippets are verified to run; x86_64 ones are labeled. The
design doc (docs/inline-asm-design.md) stays as the internal rationale;
this guide is the user-facing companion, linked from readme.md.
