Refactor MySTRA as a publication-only adapter

[lhparker1]

Summary

• define MySTRA as a portable ASTRA-to-MyST publication adapter rather than a project viewer
• give hidden resolved-store carriers page-unique identifiers while preserving the stable astra-store class contract
• tighten documentation and tests around the viewer/publication boundary

Verification

• npm test — 102 tests passed
• npm run build

Product boundary

• MySTRA (this PR): converts ASTRA records into portable MyST publication content and page-local resolved stores.
• astra-theme #7: renders that publication content as the paper/book experience.
• astra-viewer: owns host-neutral project inspection, inventory, record-detail, and result-viewer components.
• jupyterlab-astra: hosts the shared viewer packages inside JupyterLab without importing the publication theme.