Closes the «Observer instrument expansion v2» epic. The retro note is
the source of all #1-#19 references in commit messages; the plan is
the procedural source (with REVISION v1.1 after parallel-session rebase).
Both kept in repo for traceability of the 20-commit epic.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>