feat(filesystem): Implement complete mod loading through -mod

[Jaredl-Dev]

• Closes Implement complete mod loading through -mod #3022

Implements complete native mod loading for Zero Hour through ordered, repeated -mod arguments. Each operand accepts a BIG archive or a directory. Directory layers expose loose files and recursively load *.big archives.

Precedence is deterministic:

• Later -mod operands override earlier operands.
• Loose files override BIG archives within the same directory layer.
• Explicit mod layers override physical game-directory files and normal game archives.

The command line captures operands during startup. The file system validates and mounts the complete stack after the low-level file systems initialize and before GlobalData, GameData, and compatibility CRC inputs load. Missing or invalid operands produce a visible startup error without partially replacing the active stack.

Native-path consumers resolve winning loose mod files for Bink movies and animated Windows cursors without falling through to shadowed base assets. The FFmpeg backend reads winning movie resources through the file system, including BIG-backed resources.

Scope

This PR implements Zero Hour first. The corresponding Generals implementation is intentionally deferred until the Zero Hour code review is complete. The Generals target requires that replica before it builds with this series.

Known limitations

• Mod-provided splash screens are not loaded by this implementation. Splash screens from mod layers require separate startup work that is better handled in a separate PR.

• Mod archives whose resolved paths reach the legacy Windows 260-character limit may fail to open because the engine uses narrow-character CRT file APIs. A separate filesystem change to support Unicode extended-length paths would be required to resolve this issue.

Validation

• Build Zero Hour with modern MSVC
• Build Zero Hour Release with Visual C++ 6
• Launch Contra 10.0.2 Beta 2 Patch 1
• Launch C&C Shockwave 1.201 with the HD cursor add-on
• Run git diff --check

TODO

• Replicate to Generals