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/strings.lux

@@ -0,0 +1,15 @@
+// Benchmark: String operations (measures string performance)
+// Tests: String concatenation, conversion, manipulation
+
+fn buildString(n: Int, acc: String): String =
+    if n <= 0 then acc
+    else buildString(n - 1, acc + toString(n) + " ")
+
+fn countWords(s: String): Int =
+    List.length(String.split(s, " "))
+
+// Build a string with 1000 numbers
+let longString = buildString(1000, "")
+
+// Count words in the string
+let wordCount = countWords(longString)