docs: add demos and update documentation for new features

Documentation:
- Update IMPLEMENTATION_PLAN.md with current status (222 tests)
- Update feature comparison table (Schema Evolution, Behavioral Types: ✅)
- Add HttpServer to built-in effects list
- Update OVERVIEW.md with working behavioral types examples

Demo Programs:
- examples/schema_evolution.lux - Version annotations, constraints, runtime ops
- examples/behavioral_types.lux - pure, deterministic, commutative, idempotent, total

Sample Project:
- projects/rest-api/ - Full REST API demo with:
  - Task CRUD endpoints
  - Pattern matching router
  - JSON serialization
  - Effect-tracked request handling

Tests:
- Add behavioral type tests (pure, deterministic, commutative, idempotent, total)
- 227 tests passing

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/OVERVIEW.md

@@ -157,11 +157,32 @@ import math.{sqrt, abs}
 import prelude.*
 ```
 
+### Also Working
+
+```lux
+// Behavioral types with compile-time verification
+fn factorial(n: Int): Int is pure is deterministic is total =
+    if n <= 1 then 1 else n * factorial(n - 1)
+
+fn add(a: Int, b: Int): Int is commutative = a + b
+
+fn absolute(x: Int): Int is idempotent =
+    if x < 0 then 0 - x else x
+
+// Schema evolution with version tracking
+fn processV2(data: Int @v2): Int = data * 2
+let value: Int @v2 = 42
+let result = processV2(value)
+
+// Version constraints
+fn processModern(x: Int @v2+): Int = x  // v2 or later
+fn processAny(x: Int @latest): Int = x  // any version
+```
+
 ### Planned (Not Yet Fully Implemented)
 
-- **Schema Evolution**: Parsing works (`@v1`, `from @v1`), type system integration missing
-- **Behavioral Types**: Parsing works (`is pure`, `is total`), verification beyond `pure` missing
 - **Full Compilation**: JIT works for numeric code, strings/lists/ADTs missing
+- **Auto-migration Generation**: Migration bodies stored, execution pending
 
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