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/types.rs

@@ -1148,8 +1148,10 @@ pub fn unify(t1: &Type, t2: &Type) -> Result<Substitution, String> {
                 ));
             }
 
-            // For now, effects must match exactly
-            if e1 != e2 {
+            // Function's required effects (e1) must be a subset of available effects (e2)
+            // A pure function (empty effects) can be called anywhere
+            // A function requiring {Logger} can be called in context with {Logger} or {Logger, Console}
+            if !e1.is_subset(&e2) {
                 return Err(format!(
                     "Effect mismatch: expected {{{}}}, got {{{}}}",
                     e1, e2