ci: make release invariants follow the active API release line

[codeforester]

Goal

Make release-invariant API checks follow the active release line without hidden local environment state.

Background

tests/release-invariants.sh hard-codes v2.0.0 as the default BASE_BASH_LIBS_RELEASE_REF, while the only documented release workflow promotes future API metadata for the next 2.x release. No CI workflow sets this override, and docs/release-process.md does not describe it. When a preview module is promoted to stable for a future release (for example, since: 2.1.0), the validation job will still compare that stable module against v2.0.0, where it cannot exist, unless a maintainer supplies an undocumented environment override. That makes release-preparation validation non-reproducible and risks either a blocked PR or an unreviewed CI configuration difference.

Scope

• Derive or explicitly configure the release reference used by release-invariants from the active release contract and candidate version.
• Keep the current mainline check against the published GA reference when no release candidate is active.
• Document the CI/local command for release-preparation branches and make the workflow use the same value.
• Add a regression fixture covering promotion of a preview module into a future release tree.

Acceptance Criteria

• A release-preparation branch for the next supported 2.x version passes the same documented command locally and in CI without an undocumented environment variable.
• Ordinary mainline validation continues to enforce compatibility with the published GA release.
• The selected release ref and its provenance are visible in failure diagnostics.

Validation

• ./tests/validate.sh
• Focused tests/release-invariants.sh/API-manifest tests for current-GA and future-release references.
• CI workflow review confirms no divergent hidden environment setup.

Non-Goals

• Changing the stable API compatibility policy.
• Publishing or retagging a release.

Project Fields

• Status: Backlog
• Priority: P2
• Area: CI
• Initiative: Adoption Polish
• Size: S
• Milestone: v2.1.0

Agent Assignment

• Assignee: codeforester