Fix outdoor fog affecting office interiors

[Sairamnaidu456]

Outdoor fog currently fades desks, workers and screens inside the office. Mask the fog by each rendered surface’s world position using the existing indoor lighting boundaries, including expanded back offices, so outdoor scenery stays foggy through the windows. Roof and alternate-map modes retain their existing fog.

Move the fog shader into its own module and lower the sky file’s size ceiling. Update the feature documentation and add regression coverage for toon, basic and sprite material shader integration.

Closes #122.

Validation:

• npm run typecheck
• npm test — 609 tests passed
• npm run build
• Headless Chrome: dense-fog office screenshot and rendered pixel assertions for lit/unlit indoor surfaces, outdoor fog and alternate-map mode; no shader errors.