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/factorial.lux

@@ -1,4 +1,6 @@
 // Factorial function demonstrating recursion
+//
+// Expected output: 10! = 3628800
 
 fn factorial(n: Int): Int =
     if n <= 1 then 1
@@ -7,6 +9,8 @@ fn factorial(n: Int): Int =
 // Calculate factorial of 10
 let result = factorial(10)
 
-// Print result
-fn main(): Unit with {Console} =
-    Console.print("10! = " + result)
+// Print result using Console effect
+fn showResult(): Unit with {Console} =
+    Console.print("10! = " + toString(result))
+
+let output = run showResult() with {}