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>

src/interpreter.rs

@@ -881,7 +881,21 @@ impl Interpreter {
                 Ok(value)
             }
 
-            Declaration::Effect(_) | Declaration::Type(_) | Declaration::Trait(_) | Declaration::Impl(_) => {
+            Declaration::Type(type_decl) => {
+                // Register ADT constructors if this is an enum type
+                if let crate::ast::TypeDef::Enum(variants) = &type_decl.definition {
+                    for variant in variants {
+                        let constructor = Value::Constructor {
+                            name: variant.name.name.clone(),
+                            fields: Vec::new(),
+                        };
+                        self.global_env.define(&variant.name.name, constructor);
+                    }
+                }
+                Ok(Value::Unit)
+            }
+
+            Declaration::Effect(_) | Declaration::Trait(_) | Declaration::Impl(_) => {
                 // These are compile-time only
                 Ok(Value::Unit)
             }