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

[zackees]

Parent tracking: zackees/soldr#922.

Current state

.github/workflows/auto-release.yml + six per-platform build files (linux-arm-build.yml, linux-x86-build.yml, macos-arm-build.yml, macos-x86-build.yml, windows-arm-build.yml, windows-x86-build.yml). Each runs on a matching native runner (macos-latest, windows-latest, ...).

Target state

One ubuntu-24.04 job family. Bootstrap soldr (from PyPI pin), then for every target:

• x86_64-unknown-linux-gnu, aarch64-unknown-linux-gnu, aarch64-unknown-linux-musl, x86_64-unknown-linux-musl: soldr cargo zigbuild --release --target <triple>
• x86_64-apple-darwin, aarch64-apple-darwin: soldr cargo zigbuild --release --target <triple>
• x86_64-pc-windows-msvc, aarch64-pc-windows-msvc: soldr cargo xwin --release --target <triple>

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

Dry-run

Add a dry-run workflow_dispatch input (or reuse if present) that runs the full build matrix but skips the publish step.

Checklist

• Bootstrap soldr step on ubuntu-24.04
• Convert each per-platform build file to a matrix entry using soldr-driven zigbuild/xwin
• Add or verify dry-run input
• Trigger dry-run from PR branch and attach the run URL to the PR
• Update README badges if any reference the old per-OS workflows