Cookie jar for HTTP client

[lalinsky]

The HTTP client is currently stateless — cookies received in Set-Cookie response headers are not stored or replayed in subsequent requests. This makes it impossible to use the client for workflows that depend on cookie-based sessions, such as testing login flows or scraping sites that require authentication.

Proposed feature

Add a CookieJar that can be optionally attached to a Client (or passed per-request via FetchOptions). It should:

• Parse and store cookies from Set-Cookie response headers
• Replay matching cookies in the Cookie request header on subsequent requests
• Respect cookie attributes: Domain, Path, Expires/Max-Age, Secure, HttpOnly
• Handle cookie eviction (expired cookies are not sent)

Example usage

var jar = http.CookieJar.init(allocator);
defer jar.deinit();

var client = http.Client.init(gpa, io, .{ .cookie_jar = &jar });
defer client.deinit();

// Login — server sets a session cookie
_ = try client.fetch("http://example.com/login", .{
    .method = .POST,
    .body = "user=foo&pass=bar",
});

// Subsequent request automatically sends the session cookie
var res = try client.fetch("http://example.com/profile", .{});
defer res.deinit();

Scope

• A new CookieJar struct in src/cookie.zig or a new src/cookie_jar.zig
• Integration into client.zig: populate jar from responses, attach cookies to requests
• Thread-safety is not required initially (same assumption as the rest of the client)