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/fibonacci_tco.lux

@@ -0,0 +1,11 @@
+// Benchmark: Tail-recursive Fibonacci (measures TCO efficiency)
+// Tests: Tail call optimization, accumulator pattern
+
+fn fibTCO(n: Int, a: Int, b: Int): Int =
+    if n <= 0 then a
+    else fibTCO(n - 1, b, a + b)
+
+fn fib(n: Int): Int = fibTCO(n, 0, 1)
+
+// Calculate fib(100000) - many iterations, constant stack
+let result = fib(100000)