Add user-info message support to RetroArch WebSocket monitor

[Copilot]

Handles the "user" and "no_user" messages introduced in Octelys/retro-arch#4, alongside the previously added "game_playing", "no_game", and "achievements" messages.

retro_achievements_monitor.h

• New retro_user_t struct:

  typedef struct {
      char     username[128];
      char     display_name[128];
      uint32_t score;
      uint32_t score_softcore;
      char     avatar_url[512];
  } retro_user_t;

• New callback types on_retro_user_t / on_retro_no_user_t
• New subscription API: retro_achievements_subscribe_user() / retro_achievements_subscribe_no_user()

retro_achievements_monitor.c

• user_subscription_t / no_user_subscription_t linked lists and notify_user() / notify_no_user() dispatch helpers
• Parses {"type":"user", "username":"...", "display_name":"...", "score":N, "score_softcore":N, "avatar_url":"..."} and {"type":"no_user"}, dispatching to subscribers
• Real and HAVE_LIBWEBSOCKETS-guarded stub implementations for both new subscription functions

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[kzryzstof]

@copilot It is now missing the code to start & stop the monitoring of RA

[Copilot]

@copilot It is now missing the code to start & stop the monitoring of RA

Done in 5c30d57. retro_achievements_monitor_start() is now called in obs_module_load and retro_achievements_monitor_stop() is called in obs_module_unload, following the same pattern as the Xbox monitor.

[kzryzstof]

@copilot In the PR Octelys/retro-arch#4, I just have added the support to send the retro-achievements as another message. Update this current branch to retrieve them.

[Copilot]

@copilot In the PR Octelys/retro-arch#4, I just have added the support to send the retro-achievements as another message. Update this current branch to retrie...

Done in 9c4971c. The monitor now handles the new "achievements" message shape from retro-arch#4:

New retro_achievement_t struct (in the header):

typedef struct {
    uint32_t id;
    char     name[256];
    uint32_t points;
    char     status[16];    // "unlocked" or "locked"
    char     badge_url[512];
} retro_achievement_t;

New callback type and subscription API:

typedef void (*on_retro_achievements_t)(const retro_achievement_t *achievements, size_t count);
void retro_achievements_subscribe_achievements(on_retro_achievements_t callback);

The monitor parses the incoming {"type":"achievements","items":[...]} message, builds the array of retro_achievement_t records, and dispatches it to all subscribers.