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/effects.lux

@@ -0,0 +1,24 @@
+// Benchmark: Effect handling (measures effect overhead)
+// Tests: Effect dispatch, handler lookup, effect calls
+
+effect Counter {
+    fn increment(): Unit
+    fn get(): Int
+}
+
+fn countTo(n: Int): Int with {Counter} =
+    if n <= 0 then Counter.get()
+    else {
+        Counter.increment()
+        countTo(n - 1)
+    }
+
+handler counterHandler: Counter {
+    fn increment() = ()
+    fn get() = 0
+}
+
+// Run the effect benchmark
+let result = run countTo(10000) with {
+    Counter = counterHandler
+}