Phase 5: federation

Implements PLAN.md Phase 5: the Server-Server API. Warp joins rooms on
other servers and lets their users join its rooms, invites across servers
with the invitee's server countersigning, rejects remote invites (locally
if the inviter is unreachable), and exchanges events, typing, receipts and
to-device messages with other homeservers — all behind the runtime switch,
which now starts and stops real outbound traffic.

The protocol lives in internal/federation with no knowledge of rooms:
X-Matrix signing (fuzzed parser), the spec's discovery algorithm keeping
the dial address and the TLS name apart, a signed client that refuses
every request while the switch is off or the policy denies the
destination, and a key ring that caches other servers' keys for at most a
week, with optional notaries.

The roomserver takes events from elsewhere through the spec's intake:
signatures and hashes (a hash mismatch keeps the redacted form), the rules
against the event's own auth events, against the state before it — with
/get_missing_events for small gaps and /state_ids for wide ones — and
against the current state, where a failure is a soft failure. Events held
without their state are outliers: a column on roomserver_events, because
the current state of a room joined over federation is almost all
outliers, and excluded from every timeline query. Forks are resolved at
the room's head. Membership across servers is make_join/send_join,
make_leave/send_leave and countersigned invites, in both directions;
m.room.server_acl is enforced on everything another server sends.
History before a remote join is backfilled when /messages pages past it,
stored at negative stream positions.

internal/federationapi is the running component: the inbound endpoints,
and an outbound queue that persists what it owes each destination, sends
it in transactions of 50 PDUs and 100 EDUs, backs off from ten seconds to
six hours, survives restarts, and drops events persisted while federation
is off rather than delivering a backlog in one burst. Its Module is what
the switch starts and stops.

Also: a TLS listener for port 8448, trusted key servers, key validity and
an outbound CA bundle in the config; per-destination health and backoff
reset in the admin API; remote profiles, aliases and directories in the
Client-Server API; a Complement federation profile. See
docs/federation.md, including its known gaps (no partial-state joins, no
key rotation).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/development.md

@@ -147,15 +147,17 @@ far along the server is — PLAN.md section 9 treats it as the primary progress
 metric.
 
 ```sh
-make complement                                   # the current target set
+make complement                                   # the C-S set, federation off
+make complement-federation                        # the federation set
 server/test/complement/run.sh TestLogin           # one test
 COMPLEMENT_DEBUG=1 make complement                # with the container logs
 ```
 
 It needs Docker. The script builds the image, clones Complement into
 `.complement/` (ignored by git) and runs the subset for the phases that are
-done — the account, room and stream tests, with federation off. Extend
-`COMPLEMENT_TESTS` in
+done. `COMPLEMENT_PROFILE` picks the set: `csapi` (the default) runs the
+account, room and stream tests with federation off; `federation` runs the
+Server-Server tests with it on. Extend the sets in
 [run.sh](../server/test/complement/run.sh) as each phase lands, rather than
 switching to the whole suite: a pass count that drops because a later phase
 is missing is noise.
@@ -165,9 +167,9 @@ and the deployment is entirely `WARP_*` environment variables — see
 [entrypoint.sh](../server/test/complement/entrypoint.sh). Two things there
 are worth knowing: rate limiting is off, because the whole suite comes from
 one address and would otherwise spend its time reading `M_LIMIT_EXCEEDED`;
-and port 8448 is exposed but nothing listens on it yet. The federation
-profile needs a TLS listener signed by the CA that Complement mounts at
-`/complement/ca`, which arrives with federation in Phase 5.
+and port 8448 is served over TLS with a certificate the entrypoint has the
+CA Complement mounts at `/complement/ca` sign, the same CA Warp is told to
+trust for its outbound connections.
 
 ## Creating the first account