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>

benchmarks/closures.lux

@@ -0,0 +1,20 @@
+// Benchmark: Closure creation and calls (measures closure overhead)
+// Tests: Closure capture, higher-order functions
+
+fn makeAdder(n: Int): fn(Int): Int =
+    fn(x: Int): Int => x + n
+
+fn applyN(f: fn(Int): Int, x: Int, n: Int): Int =
+    if n <= 0 then x
+    else applyN(f, f(x), n - 1)
+
+// Create 1000 closures and apply them
+fn benchmark(count: Int): Int =
+    if count <= 0 then 0
+    else {
+        let adder = makeAdder(count)
+        let result = applyN(adder, 0, 100)
+        result + benchmark(count - 1)
+    }
+
+let result = benchmark(1000)