bugfix(map): Accept either path separator in map lookups

[bobtista]

Closes #3103
Depends on #3105

Map validation rejects a preferred map with different path separators and falls back to the default before LAN setup can resolve it.

Now validation uses findMap, and cache keys and lookups use native separators. LAN setup uses the resolved filename. Custom-match preferences also normalize separators for callers that still use exact lookups.

Reproduction:

1. Select a non-default multiplayer map, then change its stored LAN preference to use the other separator or a mix of both.
2. Host a LAN game.
3. Check that the selected map is retained and the host has its availability, CRC, and size set.

Testing:

• macOS integration probe: old validation selects the default for reversed and mixed separators; fixed validation retains the selected map and sets the expected host metadata.
• Native separators, uppercase names, missing-map fallback, and custom-match preferences pass.
• Separator helper passes sanitizer checks under both POSIX and Windows separator conventions.