feat(validate-story): add scripts/validate-story.js (bd-ge-hch.3.4.5)

[SorraTheOrc]

Summary

• add validate-story CLI to compile and auto-walk Ink stories with seeded choice selection
• add unit tests for parse failure, success, and deterministic runs plus fixtures
• expose jest-only test script to allow unit test runs without demo flow

Files Changed

• scripts/validate-story.js
• tests/validate-story/validate-story.test.js
• tests/fixtures/valid.ink
• tests/fixtures/invalid.ink
• package.json
• package-lock.json
• jest.config.js

Testing

• npm run test:unit
• node scripts/validate-story.js --story tests/fixtures/valid.ink --seed 123 --max-steps 100 --output stdout

BD Links

• ge-hch.3.4
• ge-hch.3.4.5

Notes

• CI workflow updates, if any, remain to Ship/CI