Move embedded file table, addon registration hooks, and loading logic
from main.zig into addons.zig. Each module's is_active callback gates
addon loading on mutex ownership so individual DLLs only load addons
for modules they control. Addon hooks are skipped entirely at comptime
when no addon-bearing modules are compiled in.
Also consolidates CheckFileExistence hook — customassets no longer hooks
it directly, instead exporting looseFilesLookup for main.zig's hook.
