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/list_operations.lux

@@ -0,0 +1,18 @@
+// Benchmark: List operations (measures collection performance)
+// Tests: List creation, map, filter, fold
+
+fn square(x: Int): Int = x * x
+fn isEven(x: Int): Bool = x % 2 == 0
+fn add(acc: Int, x: Int): Int = acc + x
+
+// Create a list of 10000 elements
+let numbers = List.range(1, 10000)
+
+// Map: square each element
+let squared = List.map(square, numbers)
+
+// Filter: keep only even numbers
+let evens = List.filter(isEven, squared)
+
+// Fold: sum all values
+let sumResult = List.fold(0, add, evens)