feat: add RSS 2.0 feed generation support

[gumruyanzh]

Summary

• Adds jetzig.Rss module for generating valid RSS 2.0 XML feeds
• Provides Channel and Item structs with proper XML escaping
• Exports via jetzig.Rss so views can use request.renderText() to serve feeds
• Includes demo view (demo/src/app/views/feed.zig) showing the pattern

Design

Rather than middleware, this takes the utility-module approach: views generate RSS XML using the builder and return it via request.renderText() with content_type = "application/rss+xml". This is consistent with how the framework already handles custom content types (see render_text.zig demo).

Example usage

const jetzig = @import("jetzig");

pub fn index(request: *jetzig.Request) !jetzig.View {
    const channel = jetzig.Rss.Channel{
        .title = "My Blog",
        .link = "https://example.com",
        .description = "Latest posts",
        .items = &.{
            .{ .title = "First Post", .link = "https://example.com/first", .description = "Hello" },
        },
    };
    const xml = try jetzig.Rss.generate(request.allocator, channel);
    request.response.content_type = "application/rss+xml";
    return request.renderText(xml, .ok);
}

Test plan

• zig build succeeds
• zig build test --summary all - all 75 tests pass
• zig test src/jetzig/Rss.zig - 3/3 RSS unit tests pass (minimal feed, items, XML escaping)

Closes #36

🤖 Generated with Claude Code