Add assistant recall benchmark gate

[tsotchke]

Summary

• add ASSIST.EXE --recall-probe for retrieval-only KB2/KDB timing across all shipped packs
• add a host benchmark parser/report and wire it into assistant, hardware-capture, capability-report, and preview-release gates
• refresh QEMU hardware-capture evidence with ARECALL.LOG and a recall benchmark report

Verification

• .venv-torch311/bin/python -m unittest discover tests
• QEMU_TIMEOUT_SECONDS=300 bash qemu/run_assistant_recall_486.sh
• QEMU_TIMEOUT_SECONDS=420 bash qemu/run_hardware_capture_486.sh
• python3 scripts/verify_assistant_packs.py
• python3 scripts/verify_hardware_capture.py --capture-dir qemu/evidence/hardware_capture_486_qemu
• .venv-torch311/bin/python scripts/build_preview_release.py --self-test
• .venv-torch311/bin/python scripts/verify_preview_artifacts.py
• LC_ALL=C LANG=C shasum -a 256 -c release sidecars

Evidence

• standalone QEMU recall: 42/42, avg 61 ms, max 110 ms, recall modes kb2_term=42
• hardware-capture rehearsal recall: 42/42, avg 82 ms, max 170 ms, recall modes kb2_term=42