Allow under-target quality knob followups

[shiny-code-bot]

Summary

• Treat under-target quality follow-ups as allowed when they improve quality-related video knobs, not only VMAF/XPSNR targets.
• Reuse the same quality-improvement detection in the final approval gate so approval stays in sync with preview alignment.
• Add regression coverage for under-target grain increases in preview and approval.

Verification

• uv run --with pytest pytest -q tests/test_tuning_runtime.py -k "under_target_size_budget_quality"
• uv run --with pytest pytest -q tests/test_encode_queue_recovery.py -k "under_target_quality"
• uv run --with pytest pytest -q tests/test_tuning_runtime.py tests/test_encode_queue_recovery.py
• bash scripts/pre-commit-check.sh
• uv run /Users/cbusillo/Developer/codex-skills/jetbrains-inspection/scripts/jb-inspect.py closeout --repo /Users/cbusillo/Developer/mediaforce --scope changed_files