Implement victor string builder and HUD tests

[themuffinator]

Summary

• add a dedicated victor string builder covering team and FFA scenarios
• wire victor resolution into all intermission HUD displays so winners are consistently shown
• add unit tests validating the intermission victor string helper

Testing

• g++ -std=c++17 src/p_hud_victor.cpp src/p_hud_victor_tests.cpp -o /tmp/p_hud_victor_tests
• /tmp/p_hud_victor_tests

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Codex Task

[chatgpt-codex-connector]

💡 Codex Review

https://github.com/themuffinator/muffmode/blob/f316a62d025fb373398a9c62ee4604fb7445ba80/src/p_hud_victor.cpp#L20-L24
Preserve queued intermission victor messages

The new BuildIntermissionVictorString zeroes the output buffer (buffer[0] = '\0';) before checking context.existing_message, but callers pass the same pointer for both (level.intermission_victor_msg). This means any message previously queued in QueueIntermission (timelimit hit, admin-forfeit, mercylimit, etc.) is cleared and the “existing message” branch never runs, so the HUD now replaces the queued reason with auto-computed winner text. Intermission screens will therefore display the wrong end-of-match reason whenever a message was set ahead of time.

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