Document save data shape and version rules

[keepithandy]

Goal

Write down the current save data shape so future engine changes have a clear contract.

Current behavior

Depth Engine saves browser state and repairs a few older field names, but the save structure is not documented in one dedicated place yet.

Proposed work

• Document the current saved fields.
• Include version, currentStage, maxStage, player, inventory, equipment, log, and completed.
• Explain the older stage field names the engine still accepts.
• Clarify that this issue should not change runtime behavior.

Acceptance checks

• The save data shape is documented.
• The documented fields match the current engine behavior.
• Existing smoke tests still pass.
• No save behavior changes are included.

Guardrails

This is a documentation issue only. Do not rewrite save loading or stored data behavior here.