fix(images): one router's extras must not cancel the whole lab

[VizzleTF]

docker compose build puts every router in one buildkit solve, and buildkit cancels the
solve on the first target that fails. The extras step ran under set -eu with no guard,
so one unusable staged file failed the whole lab.

Measured on a three-router stand — an .ipk with an unsatisfiable dependency:

#14 ERROR: … did not complete successfully: exit code: 255
#16 CANCELED
#13 CANCELED
target bad2410: failed to solve: …
owlab: build failed: exit status 1

Both release lines do it: the same stand with an .apk that apk refuses
(unable to select packages: breaks: luci-base-…[ucode>=2026.02.27]) exited 27 and
cancelled its neighbour. It is set -eu, not the package manager.

Neither docker compose build (v5.1.2) nor docker buildx bake (v0.33.0) has a
--keep-going, so tolerance has to live in the RUN itself.

Tolerance is not silence

The step installs the set in one command as before — that is what lets the package
manager resolve among the staged files. Only if that fails does it retry them one at a
time, in staged order, which is the order the developer wrote them in: a dependent
package staged after the one it needs installs second and resolves against a router that
already has it.

What still will not install is named, not swallowed:

• in the build log: owlab: FAILED to install: <file>;
• recorded in /etc/owlab/extras-failed inside the image;
• under the owlab up ready table: ! bad2410 is running WITHOUT owlab-bad-extra_1.0_all.ipk,
  with owlab up --rebuild bad2410 as the next step;
• owlab test fails the router on a new extra_packages step, before any assertion runs.

Extras are files the developer staged and explicitly asked for. Skipping one quietly and
returning a green build would be worse than failing — a check that could not run counts
as failed here, not as skipped.

After

Image owlab-lab12-bad2410:latest  Built
Image owlab-lab12-ok2512:latest   Built
Image owlab-lab12-ok2410:latest   Built
#11 4.707 owlab: FAILED to install: owlab-bad-extra_1.0_all.ipk
! bad2410 is running WITHOUT owlab-bad-extra_1.0_all.ipk
EXIT=1

No CANCELED. The healthy path is unchanged: the same stand with working extras exits 0
with no ! line, the packages are installed, and a stale extras-failed from an earlier
build is cleared at the top of the layer — this Dockerfile also runs on top of an
already-built owlab image.

Contract change

owlab up can now exit 1 with the lab running. That is deliberate and documented in
docs/reference*.md, docs/troubleshooting*.md and the changelog: a router missing a
package the developer asked for is not a successful run, and the lab is still up so it
can be inspected.

Fixes #12