Squadra 3+ 3.12.1

Squadra 3.12.1 fixes the dashboard on registries that replaced windows.

• Dashboard fix. Since 3.11.1, squadra replace records the successor tasks, reads and starts inside one replace event. The dashboard's task progress ignored those nested operations, so every registry with a replacement failed with KeyError: '<task>-takeover-…'. Task progress now applies them the way the registry does, and a regression test pins it.
• The plugin bridge copies of scripts/registry.py carry the same fix; the memoranda reader pin accepts the 3.12.1 bytes.

This release also carries everything in 3.12.0 and 3.11.2, which were tagged without a GitHub release: see CHANGELOG.md.

Python 3.12+ is required.

Validation: 2200/2200 tests passed on the exact release commit with the strict parallel runner (no known reds accepted), zero failures, errors, skips or worker errors. The regression tests failed by assertion on v3.12.0 and pass on this release. The fix and the release were each read and accepted by an independent Codex window that did not write them; one reader also checked the fix read-only against a real registry that contains a replacement.