🔧 Fix peek expression terminator issues and add ternary operator support

- Fix LLVM IR terminator missing in peek expression entry blocks
- Add proper jump instructions between peek blocks
- Implement ternary operator (? :) as syntactic sugar for peek
- Update Python LLVM externcall handling for improved compatibility
- Add comprehensive test cases for peek and ternary expressions
- Update language guide with ternary operator documentation

ChatGPTが頑張って修正してくれたにゃ！🐱

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

docs/guides/language-guide.md

@@ -15,6 +15,39 @@ Common Constructs
 - Null-coalesce: `x ?? y` → `peek x { null => y, else => x }`
 - Safe access: `a?.b` → `peek a { null => null, else => a.b }`
 
+Minimal Examples
+- Ternary
+  ```nyash
+  static box Main {
+    main(args) {
+      local a = 3
+      local b = 5
+      // Nested ternary is supported
+      local v = (a < b) ? ((b < 0) ? 40 : 50) : 60
+      return v
+    }
+  }
+  ```
+- Peek as expression block (last expression is the value)
+  ```nyash
+  static box Main {
+    main(args) {
+      local d = "1"
+      // Each arm can be a block; the last expression becomes the value
+      local dv = peek d {
+        "0" => { print("found zero") 0 }
+        "1" => { print("found one") 1 }
+        else => { print("other") 0 }
+      }
+      return dv
+    }
+  }
+  ```
+
+must_use Notes
+- Peek arms are expressions. When using a block arm `{ ... }`, the last expression is the resulting value; statements without a final expression yield no usable value.
+- Ternary is an expression; ensure both branches are type-compatible at MIR level (e.g., both yield integer or both yield string handle in current phase).
+
 When you need the implementation details
 - Tokenizer: src/tokenizer.rs
 - Parser: src/parser/expressions.rs, src/parser/statements.rs