Remove the co-op of 0.8.0: its runtime layer, relay, menu tab, settings, tests and docs.

The hidden beta shared story progress through a relay a player had to run and never showed the other players. A new co-op replaces it. Gone are runtime/coop*.c and their headers, the menu's Co-op tab and its texts in four languages, tools/coop_relay.py, the co-op tests, docs/coop.md, the [coop] section of nx2navi.ini.example, the --probe-coop mode, the coop trace item, and Monocypher, which only this layer used.

The old settings stay known as retired keys, so an nx2navi.ini that still sets them logs no unknown key, and they stay on the list of private settings, so a bug report keeps a room's secret, a relay's address and a name out. The platform layer's UDP sockets and fs_save_snapshot stay for the new co-op; cfg_ini_now and fs_profile_path, which nothing else called, are removed.

Checked: the game builds and starts, the menu shows its four tabs, --probe-cfg and --probe-fs pass, and the launcher's private-settings test passes. tests/save_share_check.py builds without its co-op stub; it cannot run from this worktree because the folder's path is longer than Windows allows, on main too.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

docs/architecture.md

@@ -45,8 +45,6 @@ from other projects, with its license, is listed in [THIRD_PARTY_NOTICES.md](../
   order it logged them: a line waits for the queue's lock, and one that finds the queue full is written after
   everything queued before it. A fatal error, the crash report and the end of the process write the queue out first
   (`rt_log_sync`). `NX2NAVI_LOG_ASYNC=0` writes each line where it is logged.
-- Co-op (`coop.c`, `coop_proto.c`, [coop.md](coop.md)): a host thread with a UDP socket to a relay (`tools/coop_relay.py`),
-  rooms of up to 8 players and a channel for messages between them; it never holds a virtual core.
 - OS calls the host makes go through `runtime/platform/` (`platform.h`, `win32.c`).
 
 #### Threads and cores