docs(plans): inline-asm design + ASM and FFI-linkage plans/checkpoints

Two new workstreams:
- ASM: inline assembly — asm { "tmpl", "=r" -> T, "r" = expr, clobbers(.…) },
  multi-return tuples; lowers via the existing llvm_api.c (no shim).
- FFI-linkage: add extern/export postfix keywords, migrate every #foreign onto
  them, then purge 'foreign' from the tree (end-state invariant).

Drop current/ from .gitignore so plans + checkpoints are tracked normally
(the dir was ignored; only checkpoints had been force-added). Includes
docs/inline-asm-design.md. specs.md change left uncommitted.

.gitignore

@@ -3,5 +3,4 @@ zig-out
 .DS_Store
 .vscode/
 .sx-cache
-.sx-tmp
-current/
+.sx-tmp