Fix suite proof pack case path resolution

[MiaoDX]

Summary

• resolve repo-relative suite result output_dir values before falling back to suite-relative paths
• add regression coverage for GR00T-style suite reports

Verification

• uv run ruff check src/roboharness/evidence/proof_pack.py tests/unit/evidence/test_proof_pack.py
• uv run ruff format --check src/roboharness/evidence/proof_pack.py tests/unit/evidence/test_proof_pack.py
• uv run mypy src/roboharness/evidence/proof_pack.py
• uv run pytest -q tests/unit/evidence/test_proof_pack.py tests/unit/approval/test_visual_review.py --cov-fail-under=0
• uv run ruff check . && uv run ruff format --check . && uv run mypy src/