docs: restore README context and proof artifacts

[tmusser]

What changed

• restore the four diagnostic trap outcomes that were displaced by the set-membership README insertion
• restore the companion-artifact explanation and suite navigation
• refresh ROADMAP.md, artifacts/SPEC.md, and artifacts/HANDOFF.md so they distinguish released v0.2.0 behavior from the unreleased set-membership slice
• check in examples/output/set_membership_chart.vl.json at the path already documented by the example and README
• make the set-membership example write a final newline so rerunning it does not create avoidable artifact drift
• record the cleanup in the changelog

Why

The set-membership feature landed successfully and passed CI, but its README insertion accidentally replaced existing diagnostic and suite context. The implementation also made several durable project-state files stale and documented an output artifact that was not checked in. This PR restores that context and aligns the proof trail without changing package runtime behavior.

Impact

Documentation, one generated proof artifact, and a one-line example-output consistency fix. No package source, tests, dependencies, or workflow behavior changes.

Validation

• parsed the generated Vega-Lite artifact as JSON
• verified reconciled A-only, overlap, B-only, neither, and universe counts
• verified dataset names match Altair's deterministic SHA-256 naming convention
• checked required README sections and ordering
• checked trailing whitespace and final newlines across all changed files
• GitHub Actions will run the full Python 3.10-3.13 test matrix and isolated wheel checks