[frontend-contrib S12] Generated deny-by-default procedure gateway

[rickylabs]

Part of #922

Scope

Own reviewed wave: generated per-procedure route table from requires.procedures x contract metadata at /api/plugins//; server-side auth via principal port; no blind credential forwarding; CSRF/origin, limits, timeouts, abort, manual redirects, header allowlist, audit line; streaming per metadata (AI durable chat stays on its specialized adapter).

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

Acceptance

• gate: P5 threat-model checklist item-by-item negative tests
• gate: non-granted procedure returns 404/deny (no wildcard reachability test)