Refine the paper-only ASTRA theme boundary

[lhparker1]

Summary

• restore a paper-only ownership boundary for astra-theme, leaving project inventory and graph surfaces to astra-viewer
• align MyST article/book chrome and ASTRA references with the shared brand palette, backed by token contract tests
• support page-unique astra-store carriers while retaining legacy compatibility and stable evidence-row layout

Verification

• npm test — 81 tests passed
• npm run build — article and book themes built successfully

Product boundary

• MySTRA #17: emits portable publication content and resolved stores.
• astra-theme (this PR): owns paper/book layout, typography, citations, inline ASTRA references, and publication result surfaces.
• astra-viewer: owns project inventory, inspection, record detail, and reusable viewer components.
• jupyterlab-astra: hosts astra-viewer inside JupyterLab and remains independent of the publication theme.