refactor(showcase): extract private core from public default branch

[rovsh44-glitch]

Summary

• remove code-bearing private-core trees from the public default branch surface
• keep only curated showcase docs, media, and review slices in the public repo
• replace runtime-heavy public checks with a lightweight showcase-surface gate

Validation

• public showcase surface validator passed locally
• curated export validator passed locally
• no path leaks or review-required files remained in the showcase export

Notes

• private core continues to live in the separate private repository
• public proof bundles remain published separately from the showcase repo