diagnostics: gossip route omits corrected Leaf participation counters required by C0 gate

[dirvine]

The documented corrected Leaf forwarding counters are absent from GET /diagnostics/gossip. A read-only live x0x --json diagnostics gossip response contains stats/pubsub_stages/dispatcher/recv_pump/discovery_cache_entries but no participation object. Current source on main822db3e corroborates it: gossip_diagnostics in src/server/routes/network.rs constructs no participation field, while Agent already exposes the PubSubManager participation snapshot.

This prevents applying the documented #380 C0 acceptance meter in docs/380-c0-soak-gate.md and ADR0034. The existing pubsub_stages.outbound_publish_origin.relay_bytes counts nonlocal origin, including subscribed epidemic forwarding; it is not a substitute for corrected non-subscribed participation.relay_bytes.

Scope: expose the existing snapshot through the actual route and add a regression proving field presence/semantics. Preserve Leaf/Full policy and existing fields. This is an observability gap, not proof Leaf enforcement fails or a new network egress rate. Relates to #504 efficiency measurements.

[dirvine]

The route repair is merged via PR #521 atb83c47341aae1a523d563a1780f428a18a5c6c02 after exact local Rust gates, actual Axum route positive/field-removal negative controls and all required CI passed. Keep deployed diagnostics/efficiency acceptance distinct: the running production daemon has not been upgraded, and no idle C0 soak is claimed.

[dirvine]

Closing the route-observability defect as fixed by #521 (merge b83c473), verified present on main694f03d. The actual gossip route exposes Agent::gossip_participation(), and its Axum regression asserts Leaf/Full resolution, non_subscribed_forward semantics, counter presence and equality to the underlying snapshot. I checked the retained GitHub Test Suite log: gossip_route_exposes_resolved_participation_and_c0_meters passed in 0.175s; required CI was successful, with weekly/soak jobs conditionally skipped.

This closes the API-field defect only. It does not claim a deployed daemon upgrade, idle C0 soak or efficiency acceptance; those remain tracked by #504 and the release acceptance work.