Fix infinite loop in lexer when parsing attribute expressions with operators

The lexer would hang when encountering operators like + in attribute values
(e.g., class="btn btn-" + type). Added scanAttrValuePart to handle
expression continuation with operators (+, -, *, /).

Also cleaned up debug prints from view_engine.zig and demo/main.zig.

src/examples/demo/main.zig

@@ -0,0 +1,151 @@
+//! Pugz Template Inheritance Demo
+//!
+//! A web application demonstrating Pug-style template inheritance
+//! using the Pugz ViewEngine with http.zig server.
+//!
+//! Routes:
+//!   GET /           - Home page (layout.pug)
+//!   GET /page-a     - Page A with custom scripts and content
+//!   GET /page-b     - Page B with sub-layout
+//!   GET /append     - Page with block append
+//!   GET /append-opt - Page with optional block syntax
+
+const std = @import("std");
+const httpz = @import("httpz");
+const pugz = @import("pugz");
+
+const Allocator = std.mem.Allocator;
+
+/// Application state shared across all requests
+const App = struct {
+    allocator: Allocator,
+    view: pugz.ViewEngine,
+
+    pub fn init(allocator: Allocator) !App {
+        return .{
+            .allocator = allocator,
+            .view = try pugz.ViewEngine.init(allocator, .{
+                .views_dir = "src/examples/demo/views",
+            }),
+        };
+    }
+
+    pub fn deinit(self: *App) void {
+        self.view.deinit();
+    }
+};
+
+pub fn main() !void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    defer if (gpa.deinit() == .leak) @panic("leak");
+
+    const allocator = gpa.allocator();
+
+    // Initialize view engine once at startup
+    var app = try App.init(allocator);
+    defer app.deinit();
+
+    const port = 8080;
+    var server = try httpz.Server(*App).init(allocator, .{ .port = port }, &app);
+    defer server.deinit();
+
+    var router = try server.router(.{});
+
+    // Routes
+    router.get("/", index, .{});
+    router.get("/page-a", pageA, .{});
+    router.get("/page-b", pageB, .{});
+    router.get("/append", pageAppend, .{});
+    router.get("/append-opt", pageAppendOptional, .{});
+
+    std.debug.print(
+        \\
+        \\Pugz Template Inheritance Demo
+        \\==============================
+        \\Server running at http://localhost:{d}
+        \\
+        \\Routes:
+        \\  GET /           - Home page (base layout)
+        \\  GET /page-a     - Page with custom scripts and content blocks
+        \\  GET /page-b     - Page with sub-layout inheritance
+        \\  GET /append     - Page with block append
+        \\  GET /append-opt - Page with optional block keyword
+        \\
+        \\Press Ctrl+C to stop.
+        \\
+    , .{port});
+
+    try server.listen();
+}
+
+/// Handler for GET /
+fn index(app: *App, _: *httpz.Request, res: *httpz.Response) !void {
+    const html = app.view.render(app.allocator, "layout", .{
+        .title = "Home",
+    }) catch |err| {
+        res.status = 500;
+        res.body = @errorName(err);
+        return;
+    };
+
+    res.content_type = .HTML;
+    res.body = html;
+}
+
+/// Handler for GET /page-a - demonstrates extends and block override
+fn pageA(app: *App, _: *httpz.Request, res: *httpz.Response) !void {
+    const html = app.view.render(app.allocator, "page-a", .{
+        .title = "Page A - Pets",
+        .items = &[_][]const u8{ "A", "B", "C" },
+        .n = 0,
+    }) catch |err| {
+        res.status = 500;
+        res.body = @errorName(err);
+        return;
+    };
+
+    res.content_type = .HTML;
+    res.body = html;
+}
+
+/// Handler for GET /page-b - demonstrates sub-layout inheritance
+fn pageB(app: *App, _: *httpz.Request, res: *httpz.Response) !void {
+    const html = app.view.render(app.allocator, "page-b", .{
+        .title = "Page B - Sub Layout",
+    }) catch |err| {
+        res.status = 500;
+        res.body = @errorName(err);
+        return;
+    };
+
+    res.content_type = .HTML;
+    res.body = html;
+}
+
+/// Handler for GET /append - demonstrates block append
+fn pageAppend(app: *App, _: *httpz.Request, res: *httpz.Response) !void {
+    const html = app.view.render(app.allocator, "page-append", .{
+        .title = "Page Append",
+    }) catch |err| {
+        res.status = 500;
+        res.body = @errorName(err);
+        return;
+    };
+
+    res.content_type = .HTML;
+    res.body = html;
+}
+
+/// Handler for GET /append-opt - demonstrates optional block keyword
+fn pageAppendOptional(app: *App, _: *httpz.Request, res: *httpz.Response) !void {
+    const html = app.view.render(app.allocator, "page-appen-optional-blk", .{
+        .title = "Page Append Optional",
+    }) catch |err| {
+        res.status = 500;
+        res.body = @errorName(err);
+        return;
+    };
+
+    res.content_type = .HTML;
+    res.body = html;
+}