Organize RAVEL navigation and evidence documentation

[epi13]

What changed

• replaced the sprawling RAVEL landing page with a concise status-oriented entry point;
• added a documentation hub, project map, evidence guide, and version history under case-studies/ravel/docs/;
• added case-studies/ravel/tools/README.md to distinguish evaluators, digest tools, evidence writers, and the 0.6 candidate derivation;
• grouped verification commands separately from commands that rewrite evidence;
• documented why frozen versioned artifacts remain at their existing paths instead of being moved for appearance alone; and
• updated the repository-level case-study label from RAVEL 0.1–0.5 to 0.1–0.6.

Why

The RAVEL directory mixes mechanisms, contracts, preregistrations, raw observations, derived evidence, manifests, assurance cases, postmortems, development plans, and support tooling. That makes the project difficult to enter and makes distinct authority surfaces look interchangeable.

This PR adds a human navigation layer while preserving the exact filenames and locations of frozen evidence. It improves organization without invalidating historical identities or changing implementation behavior.

Impact

• no C, Python, JSON evidence, manifests, contracts, or Make targets are modified;
• no frozen artifact is renamed or relocated;
• preserved 0.4 and 0.5 FAIL results and 0.6 UNKNOWN/unauthorized boundaries remain explicit; and
• future explanatory documentation now has a clear home under docs/.

Validation

• compared the branch against main and confirmed the diff is documentation-only;
• verified referenced result, postmortem, manifest-spec, tooling, and planning filenames against the current repository; and
• no executable tests were run because runtime and evidence-producing files are unchanged.

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