docs: split-domain handles idea + interop constraint for the trails check

Prompted by Hollo's split-domain setup. Two parts:

- docs/ideas/split-domain-handles.md: offering split handle/server
  domains to self-hosters is config-level work — Fedify's origin
  option natively accepts { handleHost, webOrigin } — and handles are
  permanent identity, so apex handles matter. Single-domain stays the
  default (Decision #18); post-launch polish.
- Interop constraint pinned in social-federation task 6.1 and
  route-federation's gating decision: the trails-to-trails NodeInfo
  check must run against the actor IRI's host after WebFinger
  resolution, never the handle's domain — split-domain remote
  instances would otherwise be wrongly refused.

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

openspec/changes/route-federation/design.md

@@ -71,6 +71,11 @@ activities are neither delivered to nor accepted from non-trails
 instances. (The Wanderer-interop idea would widen this allowlist —
 explicitly out of scope here.)
 
+Split-domain interop: the check runs against the **actor IRI's host**
+(server domain) after WebFinger resolution, never the handle's domain —
+remote instances may run Hollo/Mastodon-style split domains. See
+`docs/ideas/split-domain-handles.md`.
+
 ### Decision: Sync check is a pg-boss cron, owner-driven re-fetch
 
 Daily job on the *mirror-holding* instance: for each mirror whose