docs(demo): add m2 asset readiness checks

[gowdaharshith1998-lang]

Summary

• add a Phase 8 asset validator for docs/m2_demo.cast, sample passed/failed receipts, and public docs
• add draft M2 demo and YC brief docs that explicitly remain pending until real live-run artifacts exist
• add an operator rehearsal script for the M2 demo receipt samples

Important note

• This does not fabricate docs/m2_demo.cast or docs/m2_demo_receipt_sample_*.json. The dispatch requires those artifacts to come from a real operator live run after Phase 7 and receipt samples exist.
• Running python scripts/validate_m2_demo_assets.py . on the repo currently fails as expected because the live cast and receipt samples are absent.

Verification

• pytest tests/ -q --tb=line -> 753 passed, 6 skipped, 27 xfailed
• pytest tests/docs/test_validate_m2_demo_assets.py tests/test_no_marketing_superlatives.py -q -> 5 passed
• python -m ruff check scripts/validate_m2_demo_assets.py tests/docs/test_validate_m2_demo_assets.py
• python -m mypy scripts/validate_m2_demo_assets.py
• git diff --cached --check
• npx gitnexus detect-changes --scope staged -> no indexed changes detected

Notes

• GitHub Actions may remain blocked by repository billing/quota state observed on the earlier stack.