perf(grc): materialize dashboard evidence counts

[jonathanhaaswriter]

Summary

• maintain per-runtime finding evidence counts for dashboard reads
• scope runtime selection to the authorized application workspace
• bound per-finding count serialization to the rendered preview

Validation

• make test
• make check-structural check-structural-test check-arch
• focused internal package lint and tests
• dashboard aggregate SQL-shape and optional PostgreSQL plan tests