fix: rotate bounded retention size scans

[codeforester]

Summary

• Persist an advisory byte-scan cursor in the atomic diagnostic run index.
• Rotate bounded size walks across invocations and process restarts, advancing past unreadable entries.
• Keep deletion based on live filesystem discovery and metadata/liveness revalidation, never the cursor/index.
• Cover 1,025 bundles over three fresh passes with protected early bundles and an oversized tail bundle.

Closes #355

Dependency

This PR is intentionally stacked on #366 (issue #354) so retention lease hardening merges first.

Validation

• uv run --extra dev --extra typer --extra quality python -m pytest tests/test_run_bundle_retention.py -q
• uv run --extra dev --extra typer --extra quality python -m mypy --strict lib/python/base_cli/_runtime.py
• Ruff check/format and git diff --check