M4a spec: catch-up, recording and state writes, as settled while planning

Which channels are caught up (direct channels with allowed users, allowed channels; a new one
starts from now), what posts?since returns at v11.11.0 (changed posts, only those in `order`,
at most 1,000), that a post is recorded before it is acted on, that a failed state write stops
gatewayd, and that "interrupted" is posted on the first connection after a start only.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/specs/2026-09-23-m4a-gateway.md

@@ -1,10 +1,12 @@
 # M4a design: `gatewayd`, conversations over Mattermost
 
 Status: approved by the owner, 2026-09-23, with two changes folded in (a warning for a file
-secret; channels shared with other agents, section 7). M4 is split in two (`docs/decisions.md`): M4a is the
-conversation path (this document), M4b is approvals over Mattermost (summarised in section 12,
-specified after M4a is built). Where this document and `docs/design.md` disagree, the brief wins;
-the one change it needs, P15 on secrets, is proposed in `docs/decisions.md`.
+secret; channels shared with other agents, section 7). Refined while planning, the same day:
+section 9 now says which channels are caught up, what `posts?since` returns, when a post is
+recorded, and that a failed state write stops `gatewayd`. M4 is split in two (`docs/decisions.md`):
+M4a is the conversation path (this document), M4b is approvals over Mattermost (summarised in
+section 12, specified after M4a is built). Where this document and `docs/design.md` disagree, the
+brief wins; the one change it needed, P15 on secrets, is applied (`docs/decisions.md`).
 
 Facts this rests on, checked on 2026-09-23 against the owner's server and against Mattermost's API
 source and server source at the server's version, v11.11.0:
@@ -278,13 +280,25 @@ every change:
 - `in_flight`: turns sent to `loopd` and not yet answered.
 
 A file that exists but cannot be read or parsed stops `gatewayd`, with
-`see docs/runbook.md#gateway-state-damaged`; a missing file is a first start.
+`see docs/runbook.md#gateway-state-damaged`; a missing file is a first start. So does a write that
+fails while running: going on without a record would answer posts twice after the next restart.
+
+Only posts in tracked channels are recorded: direct messages, and channels in `allow.channels`. A
+tracked post is recorded as handled **before** it is acted on, so a crash between the two loses
+the post rather than answering it twice; the `in_flight` entry reports the loss.
 
 **Connecting** (at start and after every loss): `GET /users/me`; open the WebSocket; wait for
-`hello`. Then **catch up**: for every channel in `channels`, and the direct channel with every
-allowlisted user (`POST /channels/direct`), and every channel in `allow.channels`, fetch the posts since the last one handled
-(`GET /channels/{id}/posts?since=…`) and handle them in `create_at` order, before any live event.
-On a first start, `channels` is empty and nothing is caught up: history is not answered.
+`hello`. Then **catch up**: for the direct channel with every allowlisted user
+(`POST /channels/direct`) and every channel in `allow.channels`, fetch the posts since the last one
+handled (`GET /channels/{id}/posts?since=…`) and handle them in `create_at` order, before any live
+event. A channel with no mark yet (a first start, or a user or channel newly allowed) is marked
+"now" and not caught up: history is not answered. A channel no longer allowed is not caught up.
+
+`posts?since` returns the posts **changed** after that time (Mattermost v11.11.0,
+`SqlPostStore::GetPostsSince`): only the ids in `order` changed; `posts` also holds the roots of
+their threads, which may be old; edited and deleted posts come back too. Only posts in `order`
+created after the mark and not deleted are handled. The server takes at most 1,000 changed posts,
+in no set order; a full answer is logged as "too many posts to catch up; some may be missed".
 
 **Losing the connection**: reconnect after 1, 2, 5, 10, then every 30 seconds, printing one line
 per attempt. A REST call answered 429 waits until `X-Ratelimit-Reset`; a 5xx is retried twice; a
@@ -293,7 +307,8 @@ a wrong token would not help.
 
 **Restarting**: for every entry left in `in_flight`, `gatewayd` posts "interrupted: gatewayd
 restarted before the answer arrived; ask again" in its thread and removes it. `loopd` finished the
-turn and logged it; the answer is in the session log.
+turn and logged it; the answer is in the session log. This happens on the first connection after a
+start only: after a reconnect, the turns in flight are still running.
 
 ## 10. Runbook