Add string concatenation in attributes, lazy mixin loading, and benchmarks

Features:
- Fix string concatenation in attribute values (e.g., class="btn btn-" + type)
- Lexer now properly captures full expressions with operators
- Runtime evaluates expressions for class attributes

ViewEngine improvements:
- Change mixin loading from eager to lazy (on-demand)
- Mixins are now loaded from mixins directory only when first called
- Template-defined mixins take precedence over directory mixins

Benchmarks:
- Add src/benchmark.zig with three template complexity levels
- Simple: ~150k renders/sec, 6KB memory
- Medium: ~70k renders/sec, 45KB memory
- Complex: ~32k renders/sec, 94KB memory
- Memory leak detection confirms no leaks

Documentation:
- Update CLAUDE.md with lazy mixin loading details
- Document mixin resolution order

src/tests/general_test.zig

@@ -715,3 +715,26 @@ test "Explicit self-closing tag with attributes" {
         \\<foo bar="baz" />
     );
 }
+
+// ─────────────────────────────────────────────────────────────────────────────
+// String concatenation in attributes
+// ─────────────────────────────────────────────────────────────────────────────
+
+test "Attribute with string concatenation" {
+    try expectOutput(
+        \\button(class="btn btn-" + btnType) Click
+    , .{ .btnType = "secondary" },
+        \\<button class="btn btn-secondary">Click</button>
+    );
+}
+
+test "Mixin with string concatenation in class" {
+    try expectOutput(
+        \\mixin btn(text, btnType="primary")
+        \\  button(class="btn btn-" + btnType)= text
+        \\+btn("Click me", "secondary")
+    , .{},
+        \\<button class="btn btn-secondary">Click me</button>
+    );
+}
+