Release stellar-cli 26.1.0

[github-actions]

What

Stage a new release for stellar-cli 26.1.0. builds.json is updated with the rust pairings auto-picked from the current last two minor stable releases on rust-lang/rust, and any new rust_image_digests entries are resolved.

Why

Triggered by @fnando in https://github.com/fnando/stellar-cli-docker/actions/runs/26600646689.

What is next

See RELEASE.md for the full release process.

Push any further changes to the release/v26.1.0 branch that are needed in this release (for example, adjusting the paired rust_versions if the auto-pick isn't right for this iteration).

When this PR is reviewed and merged, create a GitHub Release by going to:

https://github.com/fnando/stellar-cli-docker/releases/new?tag=v26.1.0&title=26.1.0&target=image-base-defaults

The publish workflow fires on the release-published event and:

• Builds and pushes per-arch images for any new (cli, rust base) pairs; existing pairs are skipped with a warning (per-arch tags are immutable)
• Generates SLSA build provenance + SPDX SBOM attestations on each newly-built image (buildx-native + GitHub-native chains)
• Re-points the :26.1.0 and (if newest) :latest aliases
• Attaches the SBOM and provenance files to the new GitHub Release, with per-arch digests in the body