Organize versioned RAVEL artifacts

[epi13]

Summary

• Group baseline, training, unified, 0.4, 0.5, and 0.6 material under ravel_versions/.
• Move migration and architecture-history documentation under docs/.
• Keep project metadata and the active root Makefile at the repository root.
• Add minimal compatibility runners for relocated 0.4/0.5 historical manifest paths.
• Remove committed Python cache files and add ignore rules for generated outputs.

Validation

• Python unit tests: 7 passed.
• Both study validators passed.
• Baseline, training, unified, and 0.4 Makefile checks passed.
• Unified .inc resolution passed.
• 0.4 compiler matrix, sanitizer, manifest-negative, checkpoint, lineage, and negative checks passed.
• 0.5 self-test and evaluator mutation checks passed.
• 0.6 candidate source check passed.
• Markdown link scan found no broken local links.
• Historical moved-artifact byte hashes matched the pre-move snapshot.
• Joern baseline/post relocation call-graph comparison was unchanged.

Known pre-existing limitation

The historical 0.5 manifest still references the never-imported Makefile root build specification and .github/workflows/ravel-0.5.yml. The imported case-build Makefile is preserved byte-for-byte under ravel_versions/0.5/Makefile; the missing historical dependencies were not fabricated or rewritten.