Surface actionable rust-hybrid fallback diagnostics

[jununfly]

Summary

Resolves the remaining diagnostics UX scope from #668, #669, and #670 by making degraded rust-hybrid fallback state actionable in status, doctor, and docs.

• Adds a Rust-hybrid Fallback section to human zcodegraph status output with graph usability, translated top reason groups, the exact doctor command, and the privacy-preserving artifact to share.
• Adds fallbackDiagnostics to zcodegraph status --json for scripts and agents.
• Extends degraded doctor --bundle --last-run summaries with the per-file-diagnostics.json handoff artifact and maintainer next step.
• Reuses the fallback reason label vocabulary across status and doctor so human output does not rely only on internal taxonomy codes.
• Documents the global workflow: init creates, index rebuilds, sync catches up edits, status answers trust, and doctor packages evidence.
• Records the completed roadmap under docs/plans/2026-07-02-fallback-diagnostics-ux-roadmap.*.

Scope note

This PR closes the diagnostics transparency and workflow issues. It does not implement new Vue/TypeScript parser coverage; that should follow as a separate measurable fallback-reduction slice if needed.

Verification

• npx vitest run __tests__/rust-index-engine-cli-fallback.test.ts __tests__/rust-hybrid-doctor.test.ts __tests__/status-json.test.ts
• npm run build
• git diff --check

All passed locally.