docs: capture post-v1 fediverse enhancement ideas from the soak

Ullrich's notes from the first real federation exchange (2026-06-06/07
staging ↔ Mastodon soak), expanded with the constraints the soak
surfaced: route map images in Notes, fediverse kudos (inbound
Like/Announce), comments from the fediverse, planned routes as
Events (Mobilizon), Wanderer interop, federated explore.

The fourth note item — remote follower counted but not listed — was a
bug, fixed separately in #475.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/ideas/fediverse-enhancements.md

@@ -0,0 +1,70 @@
+# Fediverse enhancements (post social-federation v1)
+
+Pre-spec exploration. Collected during the 2026-06-06/07 staging soak —
+the first real federation exchange between trails.cool (staging) and a
+Mastodon instance — partly from Ullrich's notes, partly from what the
+soak itself surfaced. Everything here builds on the shipped v1
+foundation (keys, inbox, outbox, push delivery, dereferenceable Notes).
+
+## Near-term (high demo value, each roughly a day)
+
+### Route map images in Notes
+Attach a rendered map preview (`Document`, image/png) to outgoing
+`Create(Note)` activities so Mastodon timelines show *the route*, not
+just text. The archived `journal-route-previews` change already renders
+previews; this is plumbing, not new rendering. Single highest-impact
+small change — an activity post with a map is a self-advertising
+artifact every time someone boosts it.
+
+### Fediverse kudos
+Mastodon users can already favorite/boost federated activities; our
+narrow inbox currently drops the resulting `Like`/`Announce` silently
+(by design for v1). Counting them — a `kudos` tally per activity, "12
+kudos from the fediverse" on the activity page — extends the inbox by
+two activity types with trivially small attack surface (no content,
+just references to our own objects). Undo(Like)/Undo(Announce)
+decrement.
+
+### Comments from the fediverse
+Mastodon replies to a federated activity (`Create(Note)` with
+`inReplyTo` pointing at our object) shown as comments on the activity
+page. This deliberately opens the inbound-content door the v1 design
+kept shut, so it needs the moderation/abuse story first (per-instance
+blocklist at minimum, probably report handling). The engagement payoff
+is large; the scope is the largest of the near-term items.
+
+## Future
+
+### Planned routes as Events
+Publish group rides/hikes as ActivityPub `Event` objects → Mobilizon
+and Gancio users can RSVP from their instances. Plan in the Planner,
+announce to the fediverse, meet at the trailhead. Pairs naturally with
+multi-day routes and would make trails.cool the only outdoor platform
+with federated event organizing.
+
+### Interop with Wanderer
+[Wanderer](https://wanderer.to) is a federated trail-sharing platform
+that already speaks ActivityPub. The trails-to-trails outbound check
+(social-federation §6) keys on NodeInfo `software.name`; generalizing
+the allowlist to include Wanderer (and rebranded trails forks — an open
+question in the design doc already) could connect two outdoor networks
+instead of bootstrapping one. Requires mapping their activity/trail
+vocabulary to ours — scope unknown until we read their AS shapes.
+
+### Federated explore
+Opt-in relay (or peer crawl of trails-instance outboxes) aggregating
+public routes across instances so `/explore` shows the network's best
+routes, not just the local instance's. Pairs with `route-discovery`
+(spatial search) post-launch; the relay also gives self-hosters
+discoverability from day one.
+
+## Soak learnings that constrain these (see social-federation design.md)
+
+- Mastodon requires `attachment` arrays and tombstones deleted URIs
+  forever — every new outgoing shape should be verified against a real
+  instance early, not from the spec.
+- Inbound content (kudos, comments) must go through the async queue —
+  Mastodon's 10s delivery timeout is unforgiving.
+- The `activities.owner_id NOT NULL` question (remote rows have no
+  local owner) bites comments-from-fediverse too, not just §7 outbox
+  ingestion — resolve it once, for both.

docs/roadmap.md

@@ -88,3 +88,4 @@ See `docs/ideas/` for pre-spec explorations:
 - `social-interactions.md` — local likes + comments (the foundation the federated kudos/comments attach to)
 - `activity-participants.md` — tag co-riders on shared activities, confirm/decline, federated mentions
 - `multi-day-collections.md` — activity collections for multi-day trips (architecture open question #1)
+- `fediverse-enhancements.md` — post-v1 federation: map images in Notes, fediverse kudos, comments, Events/Mobilizon, Wanderer interop, federated explore