feat(cli): Add -loadsave option

[bobtista]

Fixes #2999
Merge after #2998

There is no command-line option for loading a save during startup. Also, in headless mode, particle and ghost dummy implementations also consume less data than their rendered implementations, while terrain visual state is only partially initialized.

Now -loadsave loads a save from the user Save directory during startup in normal or headless mode.

Renderer-owned particle, terrain visual and ghost-object blocks are omitted from headless saves and skipped when loading rendered saves headlessly.

Todo:

[x] Test normal save loaded normally
[x] Test normal save loaded headlessly
[x] Test headless save loaded normally
[x] Test headless save loaded headlessly
[x] Replicate to Generals

Notes:
When using debug build, if the save file is not found, there's a "File not found" error, then it exits cleanly. In windowed, m_shellMapOn gets set false, so the menu works normally but the shellmap stops.

[greptile-apps]

Greptile Summary

Adds command-line save loading and makes save serialization compatible across rendered and headless modes.

• Registers -loadsave <filename> and loads the selected user save during engine startup.
• Omits renderer-owned particle, terrain-visual, and ghost-object blocks from headless saves and skips them when loading rendered saves headlessly.
• Centralizes Zero Hour save/load feedback and updates the relevant GUI load paths.
• Suppresses shell, shell-map, and intro startup behavior while command-line save loading is pending.

Confidence Score: 5/5

The PR appears safe to merge because no eligible blocking failure remains from the displayed follow-up review context.

No blocking failure remains.

Important Files Changed

Filename	Overview
Generals/Code/GameEngine/Source/Common/GameEngine.cpp	Adds the base-game startup path that prepares and loads a command-line-selected save before entering the main loop.
GeneralsMD/Code/GameEngine/Source/Common/GameEngine.cpp	Adds the equivalent Zero Hour startup save-loading path.
Generals/Code/GameEngine/Source/Common/System/SaveGame/GameState.cpp	Excludes renderer-owned snapshot blocks when saving or loading in base-game headless mode.
GeneralsMD/Code/GameEngine/Source/Common/System/SaveGame/GameState.cpp	Updates Zero Hour save result handling and headless snapshot-block serialization.
GeneralsMD/Code/GameEngine/Source/GameClient/GUI/GUICallbacks/SaveLoadFeedback.cpp	Introduces shared UI feedback handling for Zero Hour save and load result codes.
GeneralsMD/Code/GameEngine/Source/GameClient/GUI/GUICallbacks/Menus/PopupSaveLoad.cpp	Routes save/load menu result handling through the shared feedback implementation.
GeneralsMD/Code/GameEngine/Source/GameClient/GUI/Shell/Shell.cpp	Prevents shell and shell-map startup while a command-line save load is pending.

Sequence Diagram

sequenceDiagram
  participant CLI as Command line
  participant Engine as GameEngine
  participant State as GameState
  participant Logic as GameLogic
  participant Shell as Shell

  CLI->>Engine: Set -loadsave filename
  Engine->>State: Read save metadata
  Engine->>Logic: Prepare single-player game
  Engine->>State: Load save
  alt Load succeeds
    Engine->>Shell: Hide shell
    Engine->>Engine: Enter main loop
  else Load fails
    Engine->>Engine: Exit
  end

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