Fix seed: 0 being ignored in dungeon generation

[LucianBuzzo]

Summary

build() currently derives the seed with stage.seed || randomSeed, which treats 0 as absent and replaces it with a random value.

This PR:

• switches seed fallback logic to only generate a random seed when seed is null or undefined
• keeps explicit falsy seeds (like 0) intact
• adds a regression test proving deterministic output for seed: 0

Why

Numeric seeds are part of the public API (string | number). 0 should be a valid deterministic seed, not silently replaced.

Validation

• npm test --silent (50 tests passed)