Phase 11-12: LLVM backend initial, semantics layer, plugin unification

Major changes:
- LLVM backend initial implementation (compiler.rs, llvm mode)
- Semantics layer integration in interpreter (operators.rs)
- Phase 12 plugin architecture revision (3-layer system)
- Builtin box removal preparation
- MIR instruction set documentation (26→Core-15 migration)
- Cross-backend testing infrastructure
- Await/nowait syntax support

New features:
- LLVM AOT compilation support (--backend llvm)
- Semantics layer for interpreter→VM flow
- Tri-backend smoke tests
- Plugin-only registry mode

Bug fixes:
- Interpreter plugin box arithmetic operations
- Branch test returns incorrect values

Documentation:
- Phase 12 README.md updated with new plugin architecture
- Removed obsolete NYIR proposals
- Added LLVM test programs documentation

Co-Authored-By: Claude <noreply@anthropic.com>

apps/ny-echo/main.nyash

@@ -28,8 +28,11 @@ static box Main {
     
     parseArgs(args) {
         local options = new MapBox()
-        
-        loop(arg in args) {
+        // foreach は未対応のため、index で明示ループ
+        local i = 0
+        local n = args.length()
+        loop(i < n) {
+            local arg = args.get(i)
             if arg == "--upper" {
                 options.set("upper", true)
             } else if arg == "--lower" {
@@ -39,6 +42,7 @@ static box Main {
             } else if arg == "--help" || arg == "-h" {
                 options.set("help", true)
             }
+            i = i + 1
         }
         
         return options
@@ -47,8 +51,7 @@ static box Main {
     processInput() {
         loop(true) {
             local input = me.console.readLine()
-            if input == null { break }  // EOF
-            
+            if input == null { return 0 }  // EOF → 関数終了に変更（break未対応MIR対策）
             local output = me.transformText(input)
             me.console.log(output)
         }
@@ -75,4 +78,4 @@ static box Main {
         me.console.log("")
         me.console.log("Reads from stdin and echoes to stdout with optional transformation.")
     }
-}
+}