Preserve historical documentation in the version selector

[ikelaiah]

Summary

Extend the versioned documentation site to the full tagged history so the version selector behaves like a time machine, matching the ChronoKit-FP approach.

• Extend \docs/versions.json\ with \1.9.0, \1.8.1, \1.8.0, \1.7.0, and \1.0.0\ (all real tags; newest to oldest)
• Auto-derive navigation for legacy releases that predate \layout.json\ from only the Markdown in their own tag
• Generate a minimal landing page when a release shipped no \index.md, linking only to that release's real documents
• Pre-flight every declared \source_ref\ with \git rev-parse\ and fail clearly when it does not resolve
• Validate newest-to-oldest ordering, unique releases, current-release membership, and per-release \source_ref\ agreement in built sites
• Document the versioned documentation policy in CONTRIBUTING.md

Historical accuracy principle: each release is built from its immutable Git tag; no current documentation is copied into old releases and sparse historical documentation is intentional (v1.0.0 renders exactly one cheat-sheet page).

No UI redesign: current modern releases keep their existing navigation hierarchy, typography, theme selector, search, banner, and page shell unchanged.

Test plan

• \ ools/test_build_docs.py\ — 11 tests incl. legacy build without layout.json, sparse single-document release, selector history/current marking
• \ ools/test_build_all_docs.py\ — 4 tests incl. unresolvable source_ref failure and mixed-era released-mode worktree build
• \ ools/test_check_built_docs.py\ — 8 tests incl. ordering, duplicates, absent current, tampered source_ref
• \ ools/test_check_docs.py, \ ools/check_docs.py, \ ools/test_docs_examples.py\
• Full released-mode multi-version build: 7 paths / 41 pages
• \check_built_docs.py\ passes on both released and development-preview sites; every version-selector target resolves