B5: Asset resolver #143

Summary

Implement the asset resolver that searches for game files in mod override directories, then the game install directory, then fallback paths.

Search Order

1. Active mod directories (in load order, last wins)
2. Game installation directory
3. Built-in fallback assets (if any)

Requirements

• Path resolution: given a relative asset path (e.g. "data/Models/Ships/Galaxy/Galaxy.nif"), find the actual file
• Support multiple game install path configurations
• Case-insensitive path matching (BC's filesystem references are inconsistent)
• TGA texture loading (stb_image)

Acceptance Criteria

• Resolves NIF model paths from a BC install directory
• Resolves TGA texture paths
• Mod directory overrides game directory
• Case-insensitive matching works
• Unit tests for path resolution

Blocks: #120