release: migrate release-auto.yml to soldr cross-compile (single linux-x86 runner family)

[zackees]

Parent tracking: zackees/soldr#922.

Current state

.github/workflows/release-auto.yml + build.yml for the Rust release path. The dozens of build-<board>.yml files are out of scope — they're embedded-target CI and don't ship as Rust artifacts.

Target state

One ubuntu-24.04 job family. Bootstrap soldr, then cross-compile every release target via soldr cargo zigbuild (Linux/macOS) / soldr cargo xwin (Windows-MSVC).

Reference: soldr's cross-compile-all-targets.yml.

Dry-run

Add a dry-run workflow_dispatch input that runs the full matrix but skips publish.

Checklist

• Bootstrap soldr step on ubuntu-24.04
• Cross-compile each release target through soldr
• Add or verify dry-run input
• Trigger dry-run on PR branch
• Confirm build-<board>.yml files are untouched