fix: make all example programs work correctly

- Add string concatenation support to + operator in typechecker
- Register ADT constructors in both type environment and interpreter
- Bind handlers as values so they can be referenced in run...with
- Fix effect checking to use subset instead of exact match
- Add built-in effects (Console, Fail, State) to run block contexts
- Suppress dead code warnings in diagnostics, modules, parser

Update all example programs with:
- Expected output documented in comments
- Proper run...with statements to execute code

Add new example programs:
- behavioral.lux: pure, idempotent, deterministic, commutative functions
- pipelines.lux: pipe operator demonstrations
- statemachine.lux: ADT-based state machines
- tailcall.lux: tail call optimization examples
- traits.lux: type classes and pattern matching

Add documentation:
- docs/IMPLEMENTATION_PLAN.md: feature roadmap and status
- docs/PERFORMANCE_AND_TRADEOFFS.md: performance analysis

Add benchmarks for performance testing.

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

examples/traits.lux

@@ -0,0 +1,45 @@
+// Demonstrating type classes (traits) in Lux
+//
+// Expected output:
+// RGB color: rgb(255,128,0)
+// Red color: red
+// Green color: green
+
+// Define a simple Printable trait
+trait Printable {
+    fn format(value: Int): String
+}
+
+// Implement Printable
+impl Printable for Int {
+    fn format(value: Int): String = "Number: " + toString(value)
+}
+
+// A Color type with pattern matching
+type Color =
+    | Red
+    | Green
+    | Blue
+    | RGB(Int, Int, Int)
+
+fn colorName(c: Color): String =
+    match c {
+        Red => "red",
+        Green => "green",
+        Blue => "blue",
+        RGB(r, g, b) => "rgb(" + toString(r) + "," + toString(g) + "," + toString(b) + ")"
+    }
+
+// Test
+let myColor = RGB(255, 128, 0)
+let redColor = Red
+let greenColor = Green
+
+// Print results
+fn printResults(): Unit with {Console} = {
+    Console.print("RGB color: " + colorName(myColor))
+    Console.print("Red color: " + colorName(redColor))
+    Console.print("Green color: " + colorName(greenColor))
+}
+
+let output = run printResults() with {}