feat(builder): every boot through the node-agent, every lost file counted, Steam cache

What a builder boot sees is now what a node does:
- First boots (untouched, or -as-deployed with exactly a new server's
  settings) are deployed by the node-agent's BootTest in keep mode; the
  builder only watches the console and inspects what's left. The Go copy of
  the deploy (its own docker run, mounts and startup prefix) is gone. It had
  drifted: it gave the image's VOLUME folders their own volumes, which nodes
  don't, so saves there looked safe while every start loses them.
- Files written outside /server and /data are counted whatever their type
  (it only counted config-like extensions, so .sav or .db saves went
  unseen), including the image's VOLUME folders; logs, pid and lock files
  and system folders are left out.
- Boot tests ask the server to stay up holdSec (60 s, test.yaml can set it)
  after its ready line and to listen on every published container port
  (needs the node-agent's matching BootTest).
- Steam apps are downloaded once per build into a gbr.cache volume and copied
  into each boot, so later boots only check the files (STEAM_CACHE=off).
- RUNNER.md no longer suggests docker system prune, which deletes the
  gbr-boottest network when nothing is using it.

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

AUTHORING.md

@@ -90,6 +90,8 @@ Know this before writing anything:
 - `failPatterns`: lines that mean the server will never come up (e.g. an unaccepted EULA).
 - No reliable ready line: leave `readyPattern` out, and the boot test checks the server stays up
   (`settleSec` in `test.yaml`, default 120 s).
+- After its ready line the server must stay up for `holdSec` (default 60 s), and then listen on every port in
+  `defaultPorts`. A server that binds other ports, or only `127.0.0.1`, fails: players can't reach it.
 
 ### `test.yaml`
 - `config`: only what the server can't start without in CI (accepting the EULA, for example).