Document six-sacrifice Dragon Egg ritual

[xefensor]

What changed

• added a dedicated design document for the Dragon Egg ritual
• documented the six consumed sacrifices and their guardians/sources
• recorded Nether Reactor Core and Lodestone as working Fire/Earth artifact choices
• documented Wildfire's Stage 2 Nether role and the custom Earth guardian's terrain-building/mining mechanic
• updated the design overview, documentation index, and public roadmap to match the new direction

Why

The repository still described the progression as a four-element ritual and the current implementation only recognizes temporary Water/Air element items. This PR records the current intended design clearly before implementation changes begin.

Scope

Documentation only. No gameplay code or data behavior is changed.