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/sync.md

@@ -71,6 +71,11 @@ The spec says only that the token is opaque, and clients treat it that way.
 It is worth being readable anyway: a token is the first thing in every bug
 report about `/sync`.
 
+The PDU position is the one that can be negative: `/messages` pages back
+into history backfilled from other servers, which is numbered downwards
+from -1 (see [federation.md](federation.md#history-backfill)), and its
+tokens name positions there — `s-12`. The other streams start at zero.
+
 `/messages` and a room's `prev_batch` hand out a PDU-only token. Both are
 about room events alone, and a token from either is a valid `?since=` for
 `/sync`, where the other streams then read as unseen — the conservative
@@ -112,6 +117,12 @@ type TimelineRequest struct {
 Low exclusive and High inclusive, **in both directions**. Direction decides
 only which end the limit trims and which order the store reads in.
 
+Zero is "unbounded" at either end, and is never a position: live events
+count up from 1 and backfilled history counts down from -1. So "is this
+bound set" is `!= 0`, never `> 0`, and the position just below an event is
+`storage.PositionBefore` rather than a subtraction — below 1 is -1, because
+a token of zero handed back as a High bound would read the whole room again.
+
 Spelling the bounds as "from" and "to" instead — which is how the
 Client-Server API spells them — would mean each of them being the low bound
 in one direction and the high bound in the other, which is exactly the
@@ -354,8 +365,8 @@ for a key share is both useless and a leak.
 A message to a user of another server is dropped rather than refused. A
 client encrypting for a room with remote members sends one message per
 device in one request, and failing the request because federation is off
-would stop it from delivering to the local devices as well. Phase 5 routes
-them.
+would stop it from delivering to the local devices as well. With federation
+on, they go to the federation queue as `m.direct_to_device` EDUs instead.
 
 ### Presence
 
@@ -391,7 +402,7 @@ no timestamp and have to decide what that meant.
 |---|---|---|
 | `unread_notifications`, `unread_thread_notifications` | push rules | 8 |
 | `device_lists.changed` / `.left`, `device_one_time_keys_count` | the keyserver | 7 |
-| Remote presence, remote receipts, remote typing | federation | 5 |
+| Remote presence | presence over federation | — |
 | Threads, and threaded receipts beyond storing `thread_id` | threads | 10 |
 | `contains_url`, `event_fields` | nothing; they are parsed and ignored | — |
 | Per-event history visibility | state storage (open question 1) | — |