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/effects.lux

@@ -1,4 +1,9 @@
 // Demonstrating algebraic effects in Lux
+//
+// Expected output:
+// [info] Processing data...
+// [debug] Result computed
+// Final result: 42
 
 // Define a custom logging effect
 effect Logger {
@@ -20,16 +25,12 @@ handler consoleLogger: Logger {
     fn getLevel() = "debug"
 }
 
-// A handler that ignores logs (for testing)
-handler nullLogger: Logger {
-    fn log(level, msg) = ()
-    fn getLevel() = "none"
-}
-
-// Main function showing handler usage
+// Run and print
 fn main(): Unit with {Console} = {
     let result = run processData(21) with {
         Logger = consoleLogger
     }
-    Console.print("Final result: " + result)
+    Console.print("Final result: " + toString(result))
 }
+
+let output = run main() with {}