feat(sidecar): protocol 2 — file by type, a cursor feed, and bounded history

The sidecar now files a frame by its `type` and never by its `kind`. That is the
dumb-forwarder property made structural: `event` is appended to history,
`snapshot` replaces the board of its kind, `reply` is routed by `reqId`,
`control` is broadcast and kept nowhere. Ten new event kinds are no change here
at all, which is the whole point when the thing that grows fastest is the
catalogue.

A frame whose `type` this build does not know is dropped and counted, never
guessed at. Defaulting an absent one to `event` would file a BOARD as history —
the presence board appended a few thousand times, which nothing reports. The
count is on `/health` as `untyped_frames`, because the failure it diagnoses (a
plugin and a sidecar on different protocol versions, which the game link has no
handshake to catch) otherwise presents as a website showing nothing while the
game is plainly up. It caught exactly that within three seconds of first running,
against a protocol 1 plugin still live on a retired rig.

`boards` generalises protocol 1's single `server_state` row, and a database made
by protocol 1 is migrated in place: the two indexed columns are added by a
guarded `ALTER`, and the old board is carried across. Without that carry-over an
upgraded sidecar answers `204` until the game next connects, and the website
reads that as "never heard from" — losing a server it has rendered for weeks at
the exact moment somebody upgraded the bridge.

`GET /feed` is the ingest cursor: oldest first, strictly after an id, with
`lastId` and `more`. It is a separate route rather than a flag on `/events`
because one route with two orderings serves the other one to every caller that
forgets the parameter — and for the ingesting caller that means advancing its
cursor past rows it never read. Omitting `since` asks where the END is; `since=0`
is the other question entirely, and the two must not be separated by whether
somebody typed a parameter.

`[store].retain_days` (default 14) prunes events hourly. Boards are never pruned:
history grows and the present does not, and a pruned board is a server that has
never connected.

The repository also had no CI. `pr-checks.yml` runs the fmt, clippy and test
gates phases 1 and 3 have both been running by hand — a guard nothing invokes is
a guard whose state nobody knows.

44 tests pass, clippy clean at `-D warnings`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

sidecar/src/game.rs

@@ -204,10 +204,13 @@ pub async fn serve(
                     }
                     accept_handle.set(None).await;
                     // A disconnect is a fact the website should see without polling, so it rides
-                    // the same channel every other fact does. Nothing persists it.
+                    // the same channel every other fact does. Nothing persists it: it is this
+                    // process's own observation, not something the game said — which is exactly
+                    // what `type: "control"` means (PROTOCOL.md §8.1). It is synthesised here
+                    // rather than anywhere else because this is the only place that knows.
                     let _ = event_tx.send(GameEvent {
                         kind: "link.down".to_string(),
-                        value: serde_json::json!({ "kind": "link.down" }),
+                        value: serde_json::json!({ "kind": "link.down", "type": "control" }),
                     });
                 }
                 Err(e) => {