feat: implement string interpolation

Add support for string interpolation with {expr} syntax:
  "Hello, {name}!" becomes "Hello, " + toString(name) + "!"

Lexer changes:
- Add StringPart enum (Literal/Expr) and InterpolatedString token
- Detect {expr} in strings and capture expression text
- Support escaped braces with \{ and \}

Parser changes:
- Add desugar_interpolated_string() to convert to concatenation
- Automatically wrap expressions in toString() calls

Interpreter changes:
- Fix toString() to not add quotes around strings

Tests added:
- 4 lexer tests for interpolation tokenization
- 4 integration tests for full interpolation pipeline

Add examples/interpolation.lux demonstrating:
- Variable interpolation
- Number interpolation (auto toString)
- Expression interpolation ({a + b})
- Escaped braces

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/interpolation.lux

@@ -0,0 +1,39 @@
+// Demonstrating string interpolation in Lux
+//
+// Expected output:
+// Hello, Alice!
+// The answer is 42
+// 2 + 3 = 5
+// Nested: Hello, Bob! You are 25 years old.
+// Escaped braces: {literal} and more {text}
+
+// Basic interpolation
+let name = "Alice"
+let greeting = "Hello, {name}!"
+
+// Number interpolation (auto-converts with toString)
+let answer = 42
+let message = "The answer is {answer}"
+
+// Expression interpolation
+let x = 2
+let y = 3
+let math = "{x} + {y} = {x + y}"
+
+// Multiple interpolations
+let person = "Bob"
+let age = 25
+let intro = "Nested: Hello, {person}! You are {age} years old."
+
+// Escaped braces (use \{ and \})
+let escaped = "Escaped braces: \{literal\} and more \{text\}"
+
+fn printResults(): Unit with {Console} = {
+    Console.print(greeting)
+    Console.print(message)
+    Console.print(math)
+    Console.print(intro)
+    Console.print(escaped)
+}
+
+let output = run printResults() with {}