Until now every finished run said only `ended` or `failed`. A user stop,
a trial window running out, an idle timeout, a host restart and a game
quitting were indistinguishable afterwards.
**Schema.** Migration `0026_session_end_reason` adds two nullable
columns to `session`:
- `end_reason`: one of `user`, `window`, `hours`, `idle`, `left_queue`,
`game_exited`, `launch_failed`, `host_stopped`, `host_restart`.
- `time_claimed`: when a host took the run, set on `requested →
starting`.
**API.**
- `POST /session/:id/state` takes an optional `endReason`. With `ended`
or `failed` it is stored. With `starting` or `live` it is a 400 and the
run does not move.
- Leaving it out of a terminal report stores null, so host agents that
predate this keep working.
- `endReason` and `timeClaimed` are returned on every session response.
- The API writes the reasons it decides itself: `user` when a run nobody
has claimed is stopped, and `left_queue` when a waiting run is abandoned
(the `Left the queue` message is kept).
**New transition, `starting → ended`.** A run stopped while its game is
still downloading has failed at nothing, and never reached `live`, which
is where billing starts. Until now the only way out of `starting` was
`failed`.
**Release order: this goes first.** The host agent that sends
`endReason` is a separate change. This route's body schema is strict, so
without this deployed the current API answers that agent with a 400.
**Tests.**
- API: 185 pass, 5 of them new.
- Core: 41 session tests pass, 3 of them new.
- The new tests fail when the implementation is broken on purpose.
- No new type errors.
A host holds only so many runs at once, and until now every waiting run
was handed to it regardless; the ones it could not hold failed there.
The host now says on its heartbeat how many runs it holds, and is handed
only as many waiting runs as it has free. The rest wait in line: paid
runs before free-weekend runs, then first come first served. Reading a
waiting run returns its place in line, and says when a paid run that
asked later has gone ahead, so a free player is never reordered quietly.
A waiting run nobody has asked after for a minute was abandoned, and
leaves the line rather than holding up everyone behind it. A host that
does not report its slots is handed every run, as before.
Wanjohi