#595 ci: park Ubuntu 26.04 release host support

Status: parked — do not merge yet

This PR preserves the Ubuntu 26.04 release-host experiment from #572 for later evaluation. It is intentionally separate from the dependency/API migration in #572 and is not ready to merge while the supported Ubuntu 24.04 golden remains the release baseline.

Changes are limited to:

• ci/runner.Dockerfile: ubuntu:24.04 → ubuntu:26.04.
• .github/workflows/release.yml: release jobs that currently use ubuntu-24.04 → ubuntu-26.04.
• .github/workflows/release-runner.yml: Linux matrix hosts and runner-image host ubuntu-24.04 → ubuntu-26.04.

This branch is based on current main after merged #572 and #594; its only diff is the three Ubuntu 26.04 host/image files listed above. Do not merge this parked PR until Ubuntu 26.04 support is intentionally enabled and its image/golden compatibility is validated. #572 links here after its 26.04-only changes were removed.

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Summary by cubic

Parks the Ubuntu 26.04 release-host experiment from #572 for later evaluation without affecting the current Ubuntu 24.04 release baseline.

• Switches the runner Dockerfile base image and all Linux release job hosts from ubuntu-24.04 to ubuntu-26.04.
• Do not merge until Ubuntu 26.04 support is intentionally enabled and its image/golden compatibility is validated.

Summary by CodeRabbit

• Chores
  • Updated the Linux environments used to build, run, and publish releases to Ubuntu 26.04. Linux builds continue to use Zig.
  • These changes affect the release process only; no product features or user-facing behavior have changed.