[frontend-contrib S5] P5 proof: gateway threat model + streaming abort

[rickylabs]

Part of #922

Scope

Threat model for the deny-by-default procedure gateway (confused-deputy, header hygiene, CSRF/origin, limits, redirects) + SSE abort/reconnect semantics proven on one real plugin procedure.

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

Acceptance

• gate: written threat model reviewed in the epic
• gate: SSE abort/reconnect fixture passes