[Feature] Allow world_preset.toml seed to be omitted for random new-world seeds

[yunnwi]

Problem / Motivation

Experience-authored world_preset.toml currently appears to require or encourage a fixed seed.

That is useful for deterministic examples, repros, tests, and curated modpacks, but it is not ideal for normal player-facing world creation.

In most sandbox games, creating a new world without manually entering a seed gives a random seed. If the user wants determinism, they can explicitly provide a seed.

For Freven, a better long-term contract would be:

• seed present: use that deterministic seed
• seed omitted: generate a random seed when creating world_bootstrap.toml
• persisted world_bootstrap.toml then stores the resolved seed forever

This would let examples and games choose between reproducible worlds and normal random new-world UX.

Proposed solution

Allow the seed field in world_preset.toml to be optional.

Example deterministic preset:
[world_preset]
world_id = "example_world"
seed = 1234
primary_dimension_id = "overworld"

Example random-new-world preset:
[world_preset]
world_id = "example_world"
primary_dimension_id = "overworld"

Expected behavior when seed is omitted:

• on first world creation, DevKit/runtime generates a random seed
• the generated seed is written into world_bootstrap.toml
• future launches of the same world reuse the persisted bootstrap seed
• logs clearly show the resolved seed
• deterministic behavior remains available when seed is explicitly specified

This keeps authored presets reusable while avoiding fixed-seed behavior for normal player-facing new worlds.

Alternatives considered

1. Always require seed in world_preset.toml
   Good for reproducibility, but poor default UX for player-created worlds.

2. Make mod authors/scripts rewrite seed before launch
   Works for development, but it is not a platform-level solution and does not scale to normal users or launchers.