Add adaptive floor tiles beneath airlocks

[partyplatter08-lab]

Closes #1299.

Summary

• Spawns the existing airlock floor tile visual beneath airlocks and selects the north/south or east/west mesh based on the airlock direction.
• Copies the front and back adjacent floor materials onto the matching sides of the airlock floor tile.
• Uses a transparent fallback material when an adjacent floor is missing.
• Refreshes nearby airlocks after turf placement/removal so existing airlocks update when neighboring floors change.

Tests

• Added edit-mode coverage for airlock floor orientation and material-slot assignment helpers.
• Verified civilian and security airlock prefabs both reference the airlock floor visual and transparent fallback material.