bugfix(challenge): Fix Generals Challenge defeat during next mission intro cinematic

[coolswood]

Change

Fixes a defeat screen appearing during the intro cinematic when continuing from one Generals Challenge mission to the next via the score screen Continue button.

Root cause

startNextCampaignGame() performed popImmediate()/hideShell() before queuing MSG_NEW_GAME(GAME_SINGLE_PLAYER). Hiding the shell triggers the next menu's init (e.g. SaveLoad), which calls Shell::showShellMap(TRUE). That appends its own MSG_NEW_GAME(GAME_SHELL) for the shell map and overwrites m_pendingFile with the shell map name.

That GAME_SHELL message then races ahead of the GAME_SINGLE_PLAYER message and is processed first by onNewGame():

1. m_gameMode becomes GAME_SHELL
2. the isChallengeCampaign check in tryStartNewGame() evaluates to false (it requires m_gameMode == GAME_SINGLE_PLAYER)
3. TheGameInfo stays null
4. placeNetworkBuildingsForPlayer() is skipped
5. the local player never receives a starting base
6. the map's PLAYER_HAS_N_OR_FEWER_BUILDINGS script fires doDefeat() a few frames after the map loads, visible as the defeat screen during the intro cinematic

The actual GAME_SINGLE_PLAYER message is subsequently rejected by the onNewGame() guard (isInGame || isLoadingMap), so the wrong map/mode combination loads.

The bug only reproduces on the Continue path. Loading the same save directly via Load Game works because that path uses MSG_NEW_GAME with GAME_SINGLE_PLAYER and does not trigger the shell map race.

Fix

Reorder startNextCampaignGame() so that:

• m_pendingFile is set to the current campaign map before any shell teardown
• MSG_NEW_GAME(GAME_SINGLE_PLAYER) is queued before popImmediate()/hideShell(), guaranteeing it sits ahead of any MSG_NEW_GAME(GAME_SHELL) appended by Shell::showShellMap(TRUE)
• m_pendingFile is re-asserted after the shell teardown, since Shell::showShellMap(TRUE) may have overwritten it with the shell map name (and prepareNewGame() copies m_pendingFile into m_mapName, so the value present at queue-drain time is what actually loads)

The shell's duplicate MSG_NEW_GAME(GAME_SHELL) now lands behind ours and is rejected by the existing onNewGame() guard.

Verification

Tested on macOS (Apple Silicon), preset macos-vulkan:

• Before the fix: Load gc_americaboss -> win -> Continue -> next mission (GC_TankGeneral) loads and shows the defeat screen during the intro cinematic; the map has 0 objects owned by the local player.
• After the fix: same scenario -> next mission starts normally, the local player receives a starting base, no defeat during the cinematic.

Instrumented trace (added temporarily during investigation, not part of this PR) confirmed the message ordering: without the fix the first onNewGame() call received gameMode == GAME_SHELL; with the fix it receives gameMode == GAME_SINGLE_PLAYER and the shell's follow-up message is rejected.

Scope

Single file, single function (ScoreScreen.cpp::startNextCampaignGame()). No gameplay balance, AI or rendering changes. Applies to Zero Hour only; the Generals replica can follow in a follow-up PR per the contribution guidelines if desired.

AI generated code

The fix was authored with LLM assistance and polished/tested by the author (coolswood). The change is 21 inserted lines (one reorder plus explanatory comments) in a single function.