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/pattern_matching.lux

@@ -0,0 +1,20 @@
+// Benchmark: Pattern matching (measures ADT performance)
+// Tests: Constructor creation, pattern matching, recursion
+
+type Tree =
+    | Leaf(Int)
+    | Node(Tree, Tree)
+
+fn sumTree(tree: Tree): Int =
+    match tree {
+        Leaf(n) => n,
+        Node(left, right) => sumTree(left) + sumTree(right)
+    }
+
+fn buildTree(depth: Int, value: Int): Tree =
+    if depth <= 0 then Leaf(value)
+    else Node(buildTree(depth - 1, value), buildTree(depth - 1, value + 1))
+
+// Build a tree of depth 15 (32767 nodes) and sum it
+let tree = buildTree(15, 1)
+let sumResult = sumTree(tree)