Add readiness scores and graceful cancellation

[jacklv-coder]

What changed

• show a localized Ctrl+C cancellation hint during interactive diagnostics
• manage curl and networkQuality child PIDs so SIGINT/SIGTERM stop promptly
• return conventional exit codes 130/143, preserve cancellation feedback through redirected stderr, and clean temporary files
• add a transparent 0–100 rule_v1 developer-readiness score to human, Markdown, and JSON reports
• explicitly describe the score as a heuristic rather than a user percentile
• release as v0.6.0 because the score and JSON fields are additive features

Validation

• npm test
• bash -n bin/ipcheck test/smoke.sh
• git diff --check
• signal tests send SIGINT/SIGTERM only to the main PID, assert completion under 2 seconds, exit codes 130/143, and no temporary-directory leaks
• score coverage: 0, 35, 45, 60, 70, 75, 87, 90, 95 and all label boundaries
• final Codex CR: no remaining P1/P2 (GPT-5.6 Sol fallback; Luna unavailable in this environment)