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/behavioral.lux

@@ -0,0 +1,43 @@
+// Demonstrating behavioral properties in Lux
+// Behavioral properties are compile-time guarantees about function behavior
+//
+// Expected output:
+// add(5, 3) = 8
+// clamp(150, 0, 100) = 100
+// factorial(5) = 120
+// multiply(7, 6) = 42
+
+// A pure function - no side effects, same input always gives same output
+fn add(a: Int, b: Int): Int is pure =
+    a + b
+
+// An idempotent function - applying twice gives same result as once
+fn clamp(value: Int, min: Int, max: Int): Int is idempotent =
+    if value < min then min
+    else if value > max then max
+    else value
+
+// A deterministic function - same input always gives same output
+fn factorial(n: Int): Int is deterministic =
+    if n <= 1 then 1
+    else n * factorial(n - 1)
+
+// A commutative function - order of arguments doesn't matter
+fn multiply(a: Int, b: Int): Int is commutative =
+    a * b
+
+// Test the functions
+let sumResult = add(5, 3)
+let clampedResult = clamp(150, 0, 100)
+let factResult = factorial(5)
+let productResult = multiply(7, 6)
+
+// Print results
+fn printResults(): Unit with {Console} = {
+    Console.print("add(5, 3) = " + toString(sumResult))
+    Console.print("clamp(150, 0, 100) = " + toString(clampedResult))
+    Console.print("factorial(5) = " + toString(factResult))
+    Console.print("multiply(7, 6) = " + toString(productResult))
+}
+
+let output = run printResults() with {}