Egress interceptor: give loss a contract

itx.egress.intercept(handler) keeps its handler in a memory-only slot on the Project Durable Object. A DO restart (deploys, eviction) drops it SILENTLY: the installing session's socket stays open while every egress fetch falls through to the real egress lanes. This is the same hole the AI interceptor had before it became a live capability mount (tasks/complete/…ai-intercept-as-capability.md, PR #2528), and it has existed since egress interception shipped.

Decide and implement one of:

• Migrate egress.intercept onto the capability machinery too, like the AI interceptor — a live mount at a reserved root path, consulted from the egress path. Open question from the AI migration that matters more here: the handler contract is Request → Response (bytes, streams) — verify both cross the capability invoke leg intact.
• Or: a session-bound liveness signal for the slot. Closed PR #2527 built exactly this (a plain WebSocket whose lifetime is the DO incarnation's, closing the session 4901 on far-side loss) covering BOTH slots; its egress half can be revived from that branch (ai-interceptor-mount-invariant) nearly verbatim.

Either way, the client contract should end up the same as the AI interceptor's: an open session socket means a live interceptor; loss arrives as a close event, never silence.