Prepare v18.1.0 release notes and signposts

[flyingrobots]

Goal

Prepare the final v18.1.0 release notes and signposts before tagging main.

Scope

• Update CHANGELOG.md with concise human release notes for v18.1.0.
• Update the current public signpost documents: README.md, ARCHITECTURE.md, and docs/topics/README.md.
• Keep the consolidated docs topology intact; do not recreate docs/releases/**, docs/BEARING.md, or docs/VISION.md.
• Run release-prep validation and open a PR to main.

Acceptance criteria

• Public release summary highlights reified Optics, observer reading envelopes, bounded support planning, stream-first traversal/materialization, sync/auth, MCP, CAS encryption, and docs topology gates.
• Version metadata remains locked at 18.1.0.
• Release guard passes in prep mode on the branch.
• PR closes this issue.

[flyingrobots]

Closed by #675. The v18.1.1 release-prep PR supersedes this stale v18.1.0 signpost issue and now carries the current release notes, signpost updates, documentation topology changes, and automated release-tag flow.