Retained accounting migration evidence

See the repository retirement boundary.

This addon retains the immutable pmis.fiscal.migration.baseline model, its company rule and read-only access, and the current-fingerprint reader used by Fiscal Migration Audit. The digest layout remains compatible with the existing sealed evidence.

The old capture/sealing and period-preservation helpers, field-ownership installation hook, 18.0.2.0.0 upgrade scripts, and their obsolete tests have been removed. Migration from the former period-based implementation is no longer supported. No existing evidence is deleted or recreated. Normal accounting dates, lock protection, and authorized recognition changes remain operational.