Use async HTTP requests for UI and stats (#835)

[Eclipse1982]

Summary

Replaces the blocking HTTP requests in the UI / stats code paths with asynchronous requests, so the client no longer shows a black screen at launch (GUID hash fetch) or freezes when opening the Leaderboard and Stats menus while waiting on the stats API.

Completion callbacks run on the HTTP session thread; results are staged and marshaled to the main thread via MVC_NOTIFICATION_EVENT, and view controllers refresh in respondToEvent — following the pattern suggested in the issue and already used by NOTIFICATION_SERVER_PARSED.

Fixes #835

Changes

• Added Net_HttpGetInstanceAsync / Net_HttpGetInstancesAsync — async counterparts to the JSON instance helpers — and exposed them through the cgame import (cgi.HttpGetInstanceAsync, cgi.HttpGetInstancesAsync).
• Cl_InitGuidHash now fires an async request at startup; the hash is applied to guid_hashed on the main thread via a new NOTIFICATION_GUID_HASHED event, so startup never blocks on the network. The call was moved after Ui_Init, because MVC_NOTIFICATION_EVENT is only registered with SDL once ObjectivelyMVC initializes — a completion firing earlier would push event type 0 and the hash would be silently dropped.
• LeaderboardViewController fetches asynchronously (initial load and sort-column clicks); rows are staged under a mutex and applied in respondToEvent on NOTIFICATION_LEADERBOARD_FETCHED. It also reloads when the GUID hash arrives so the local player's row highlights.
• StatsViewController fetches asynchronously, showing placeholders until NOTIFICATION_STATS_FETCHED delivers the response; it refreshes when NOTIFICATION_GUID_HASHED arrives (previously a cold start could show "Sign in" until the next visit).
• Generation counters discard stale responses when a newer fetch supersedes an in-flight one; callbacks never touch UI state directly, so there are no dangling view controller pointers.
• Added check_http unit tests covering both new async helpers against the local test HTTP server.

Testing

• Builds without warnings (make -j$(nproc))
• Tests pass (make check)
• Tested in-game on Linux (Debian 12)

Notes

• Built and tested on Debian 12 with the from-source dependency stack from linux/docker/Dockerfile.build. The changed files compile without warnings (pre-existing warnings elsewhere in the tree are untouched).
• make check: 18/19 suites pass, including check_http with the two new async tests. The one failure is check_filesystem ("Failed to load quetoo.cfg") — an environment issue in the build container (no game data installed), unrelated to this change.
• In-game smoke test against the live stats API (giblets.quetoo.org), headless under Xvfb/llvmpipe: client reaches the main menu with stats placeholders immediately, the GUID hash arrives asynchronously and the Stats tab populates, the Leaderboard tab loads live rows, sort-column clicks refetch and reorder without any UI stall, and selectOwnRow highlights the local player after the async reload. No warnings or errors in the console log.

[jdolan]

Fantastic! Thank you. I'm working on some improvements to the JSON deserialization code here first. I'll commit that and up-merge into your branch and get this over the finish line. This is a big win for user experience.