Refresh public release surface

[tsotchke]

Summary

• refresh public README, launch, promo, video, and hardware-validation wording around legacy/constrained systems
• update assistant showcase video evidence for the regenerated era-accurate terminal demo
• refresh preview-release manifest size and expand public hygiene coverage for release-facing media sources
• update GitHub repo metadata/topics separately to match this language

Verification

• python3 -m unittest tests.test_public_repo_hygiene tests.test_assistant_showcase_video tests.test_assistant_capability_report
• .venv-torch311/bin/python scripts/build_preview_release.py --self-test
• git diff --check origin/main..HEAD

Physical hardware validation remains pending and is not claimed by this PR.