Per-world seed strategy persists

## Summary
Module setting: default seed strategy per world (random per generation / fixed string / increment from last). Honored by every generator dialog.

## Motivation
Different campaigns want different reproducibility postures. Solo prep wants "fixed; reroll until I like it"; a beta-test world wants random.

## Scope
- Setting registration in `foundry-module`.
- Strategy applied by the shared seed component (#227).

## Acceptance criteria
- [ ] Setting changes take effect on next dialog open.
- [ ] All four strategies behave correctly.