v5.0.0 — english on the wire

Every JSON key the toolkit emits is now English. This project started as a personal Spanish tool; the docs went English-first months ago but the wire keys stayed frozen in Spanish (criterios, estado, esfuerzo, hallazgos, minutos…). That inconsistency is gone:

• Evidence pack is now a11y-evidence-pack/2 — criteria, status, effort {class, minutes, why}, effort_pending, artifacts, reviewer.
• MCP arguments: reports, verified, reviewer (was informes, verificados, evaluador).
• Audit reports: findings with severity values high/medium/low, signal, remediation, summary, score_note.
• Contrast tools: verdicts, worst_ratio, area_passes_aa_normal_text_4_5, worst_zone.
• Snapshot/diff: elements, focus_order, added/removed/renamed.
• Budget/ledger: signals, blocking, history.

Stable signal identifiers (like img_sin_alt) are unchanged on purpose — they are IDs, not prose, and renaming them would invalidate existing error budgets.

This was coordinated with the Countersignatory exchange, whose draft attestation schema referenced the old keys — both sides rename in the same window, before their resolver hardens.

Seven test suites green; verified end-to-end from the stdio server.