[frontend-contrib S15] Doctor frontend check + five-state taxonomy

[rickylabs]

Part of #922

Scope

plugin doctor gains the frontend check: handshake/window, zone validity vs host descriptor, export-map presence, orphan/stale generated-output detection; prints the five-state taxonomy verbatim (unknown zone / known-but-unmounted / capacity-rejected / window-mismatch quarantine / load-failure quarantine).

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: each of the five states reproducible in a fixture and printed verbatim