release: v21.0.0 – Full Self‑Hosting (S1/S2/S3 complete)

- DoD met: S1/S2 determinism (const/compare/threeblock-collect), PRIMARY hv1 inline no-fallback, S3 (llvmlite+kernel) reps green
- Harness: v1→llvmlite direct, EXE links to libnyash_kernel.a
- Python LLVM builder fixes: cmp normalization, ret PHI synthesis, mir_call flat shape
- Using/alias polish (prod): modules-first; missing aliases added; duplicate using cleaned
- Docs: phase-21.0 COMPLETE; CurrentTask closed; release notes added

lang/src/vm/hakorune-vm/extern_provider.hako

@@ -2,20 +2,20 @@
 // Responsibility: minimal extern/global call handling on Hako side.
 // Scope (20.38準備): env.get / console.log のみ（C‑ABI導線は後段で接続）。
 
+using selfhost.vm.hakorune-vm.str_cast as StrCast
+
 static box HakoruneExternProviderBox {
   get(name, args) {
     // name: string like "env.get" or "env.console.log"
     if name == "env.get" {
       if args == null { return null }
       // args: single string key
-      using selfhost.vm.hakorune-vm.str_cast as StrCast
       local key = StrCast.to_str(args)
       return env.get(key)
     }
     if name == "env.console.log" || name == "nyash.console.log" || name == "print" {
       // Accept single argument value → print as string
       if args == null { print(""); return 0 }
-      using selfhost.vm.hakorune-vm.str_cast as StrCast
       print(StrCast.to_str(args))
       return 0
     }