✨ Implement JSON v0 Bridge with full PHI support for If/Loop statements

Major implementation by ChatGPT:
- Complete JSON v0 Bridge layer with PHI generation for control flow
- If statement: Merge PHI nodes for variables updated in then/else branches
- Loop statement: Header PHI nodes for loop-carried dependencies
- Python MVP Parser Stage-2: Added local/if/loop/call/method/new support
- Full CFG guarantee: All blocks have proper terminators (branch/jump/return)
- Type metadata for string operations (+, ==, !=)
- Comprehensive PHI smoke tests for nested and edge cases

This allows MIR generation without Rust MIR builder - massive step towards
eliminating Rust build dependency!

🎉 ChatGPTが30分以上かけて実装してくれたにゃ！

Co-Authored-By: ChatGPT <noreply@openai.com>

apps/tests/string_ops_basic.nyash

@@ -0,0 +1,15 @@
+static box Main {
+  main(args) {
+    local console = new ConsoleBox()
+    local s = "abcde"
+    // length
+    console.println("len=" + s.length())
+    // substring [1,4) -> bcd
+    local t = s.substring(1, 4)
+    console.println("sub=" + t)
+    // lastIndexOf("b") -> 1
+    console.println("idx=" + s.lastIndexOf("b"))
+    return 0
+  }
+}
+