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>

examples/await_chain.nyash

@@ -0,0 +1,18 @@
+// Chained await example
+
+static box Main {
+    main() {
+        local f1, f2, r1, r2
+        
+        // Create two futures
+        f1 = new FutureBox(10)
+        f2 = new FutureBox(20)
+        
+        // Await both
+        r1 = await f1
+        r2 = await f2
+        
+        // Return sum
+        return r1 + r2
+    }
+}

examples/await_demo.nyash

@@ -0,0 +1,17 @@
+// Await demo - demonstrates async/await functionality
+
+static box Main {
+    main() {
+        local future, result
+        
+        // Create a future with initial value
+        future = new FutureBox(42)
+        
+        // Wait for the future to complete and get value
+        result = await future
+        
+        print("Awaited result: " + result.toString())
+        
+        return result
+    }
+}

examples/await_simple.nyash

@@ -0,0 +1,14 @@
+// Simple await example for testing MIR Await instruction
+
+static box Main {
+    main() {
+        local f
+        f = me.createFuture()
+        return await f
+    }
+    
+    createFuture() {
+        // Simple future that resolves to 100
+        return new FutureBox(100)
+    }
+}

examples/await_test_vm.nyash

@@ -0,0 +1,12 @@
+// Test await without FutureBox (using integer for now)
+
+static box Main {
+    main() {
+        local value
+        value = 42
+        
+        // Note: await expects a future, but we'll test with integer
+        // This should fail, but will show if await instruction is recognized
+        return await value
+    }
+}

examples/semantics_test_array.nyash

@@ -0,0 +1,16 @@
+// Semantics test - Array operations
+
+static box Main {
+    main() {
+        local arr, value
+        arr = new ArrayBox()
+        
+        // ArrayGet/ArraySet test
+        arr.push(10)
+        arr.push(20)
+        arr.push(30)
+        
+        value = arr.get(1)  // Should be 20
+        return value
+    }
+}

examples/semantics_test_branch.nyash

@@ -0,0 +1,17 @@
+// Semantics test - Branch/Phi example
+
+static box Main {
+    main() {
+        local x, y, result
+        x = 10
+        y = 20
+        
+        if x < y {
+            result = 100
+        } else {
+            result = 200
+        }
+        
+        return result  // Should be 100
+    }
+}

examples/semantics_test_extern.nyash

@@ -0,0 +1,8 @@
+// Semantics test - ExternCall (console.log)
+
+static box Main {
+    main() {
+        print("Hello from semantics test!")
+        return 42
+    }
+}

examples/simple_test.nyash

@@ -0,0 +1,12 @@
+// Simple test without await
+
+static box Main {
+    main() {
+        local x, y, result
+        x = 10
+        y = 20
+        result = x + y
+        print("Result: " + result.toString())
+        return result
+    }
+}