Derive the coherent release route from the version bump: no per-release rebind PR #5156

Problem

Every coherent release needs a hand-made "rebind" PR before its candidate can run (#5034 for 0.114.0, and again for 0.114.1). The PR updates:

• the hard-coded version and predecessor in release-successor-candidate.yml;
• CURRENT_RELEASE in release_successor_journal.py;
• a fresh journal namespace;
• the feed-readiness observation choice;
• the pinning tests and the roadmap window section.

This costs one PR, one review and one merge per release, and it blocks any release that is otherwise ready.

Done when

• The release route derives the release version from the coherent-set version in Directory.Build.props.
• It derives the predecessor from the promoted stable-channel.json, with its source revision and content digest.
• It derives a journal namespace that is unique per version, and the feed-readiness choice per version.
• Fail-closed checks are kept:
  • the version is strictly greater than the predecessor;
  • the new version's tag and release are absent;
  • the predecessor's journal is consumed.
• A release needs no source PR beyond the version bump.
• The pinning tests become derivation tests, with mutants.
• The next release after this lands proves it, with no rebind PR.

Filed by Root/fdev as a process improvement, under the owner's standing authority. Refs #5097