[tamnd]

This change builds the default pin with GCC 16 in the kernel workflow. GCC 16 is the second reference of E11 that #114 adds.

What changes

• rk personas --for writes a second-image line when the era has a second reference. The value is the container from toolchains.toml, by digest when a digest is pinned.
• The era job of kernel.yml writes the list of compilers as JSON. For E11 the list is the era GCC, rucc and gcc-16. For an era with no second reference the list does not change.
• The build job runs rk build in the second reference's container for gcc-16. rk fetch runs on the runner first, because the container has no curl. The boot, the audits and the record run on the runner, as for the other compilers.
• The compare job still compares rucc with the first reference only. A compare against GCC 16 is a separate change.

Why

No hosted runner has GCC 16. Ubuntu 24.04 does not package it, and the toolchain PPA has only a snapshot from March. The forky container has GCC 16.2.0 from Debian, and rk personas check reads the version back.

Order

This PR replaces #116. GitHub closed #116 when the branch of #114 was deleted. #114 is merged, and the rk-era-forky image is now on the registry and public. The job uses the latest tag, as the other era containers do, until the digests are pinned.

Check

cargo test passes. On server3 I built 7.3 defconfig with the test fragment and GCC 16.2.0 in rk-era-forky in the same way. All 3076 units compiled and the kernel boots 7.3.0.

Part of #109.