chore: fix flake.nix rebuild detection

Better source change detection using find instead of single file check.

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

examples/schema_evolution.lux

@@ -1,21 +1,42 @@
 // Schema Evolution Demo
-// Demonstrates version tracking using runtime Schema operations
+// Demonstrates version tracking and automatic migrations
 
 // ============================================================
-// PART 1: Runtime Schema Operations
+// PART 1: Type-Declared Migrations
 // ============================================================
 
-// Create versioned values at runtime
-let user1 = Schema.versioned("User", 1, "Alice")
-let user2 = Schema.versioned("User", 2, "Bob")
+// Define a versioned type with a migration from v1 to v2
+type User @v2 {
+    name: String,
+    email: String,
+
+    // Migration from v1: add default email
+    from @v1 = { name: old.name, email: "unknown@example.com" }
+}
+
+// Create a v1 user
+let v1_user = Schema.versioned("User", 1, { name: "Alice" })
+let v1_version = Schema.getVersion(v1_user)  // 1
+
+// Migrate to v2 - uses the declared migration automatically
+let v2_user = Schema.migrate(v1_user, 2)
+let v2_version = Schema.getVersion(v2_user)  // 2
+
+// ============================================================
+// PART 2: Runtime Schema Operations (separate type)
+// ============================================================
+
+// Create versioned values for a different type (no migration)
+let config1 = Schema.versioned("Config", 1, "debug")
+let config2 = Schema.versioned("Config", 2, "release")
 
 // Check versions
-let v1 = Schema.getVersion(user1)  // 1
-let v2 = Schema.getVersion(user2)  // 2
+let c1 = Schema.getVersion(config1)  // 1
+let c2 = Schema.getVersion(config2)  // 2
 
-// Migrate to newer version (upgrade)
-let upgraded = Schema.migrate(user1, 2)
-let upgradedVersion = Schema.getVersion(upgraded)  // 2
+// Migrate config (auto-migration since no explicit migration defined)
+let upgradedConfig = Schema.migrate(config1, 2)
+let upgradedConfigVersion = Schema.getVersion(upgradedConfig)  // 2
 
 // ============================================================
 // PART 2: Practical Example - API Versioning
@@ -45,12 +66,16 @@ let payload2 = resp2.payload
 fn main(): Unit with {Console} = {
     Console.print("=== Schema Evolution Demo ===")
     Console.print("")
-    Console.print("Part 1: Runtime schema operations")
-    Console.print("  User v1 version: " + toString(v1))
-    Console.print("  User v2 version: " + toString(v2))
-    Console.print("  After upgrade: " + toString(upgradedVersion))
+    Console.print("Part 1: Type-declared migrations")
+    Console.print("  User before migration: v" + toString(v1_version))
+    Console.print("  User after migration: v" + toString(v2_version))
     Console.print("")
-    Console.print("Part 2: API versioning")
+    Console.print("Part 2: Runtime schema operations")
+    Console.print("  Config v1 version: " + toString(c1))
+    Console.print("  Config v2 version: " + toString(c2))
+    Console.print("  After upgrade: " + toString(upgradedConfigVersion))
+    Console.print("")
+    Console.print("Part 3: API versioning")
     Console.print("  Response v1 payload: " + payload1)
     Console.print("  Response v2 payload: " + payload2)
 }

flake.nix

@@ -31,7 +31,7 @@
 
           shellHook = ''
             # Build and add lux to PATH
-            if [ ! -f target/release/lux ] || [ Cargo.toml -nt target/release/lux ] || [ src/main.rs -nt target/release/lux ]; then
+            if [ ! -f target/release/lux ] || [ Cargo.toml -nt target/release/lux ] || [ -n "$(find src -name '*.rs' -newer target/release/lux 2>/dev/null | head -1)" ]; then
               echo "Building lux..."
               cargo build --release
             fi