Harden build cache input and diagnostics

[yoshifuminakamura]

Summary

• Hash Git-tracked files under programs/<code>/ as app-owned build cache inputs.
• Prioritize avoiding stale binary restores over maximizing build cache hit rate.
• Clarify build cache diagnostics for matched, newly recorded, and rejected cache entries.
• Add regression coverage that app-local source changes cause a cache miss and fresh store.

Tests

• bash scripts/tests/test_build_cache.sh
• .venv/bin/python -m pytest result_server/tests/test_result_detail_template.py -q
• .venv/bin/python result_server/tests/run_result_server_tests.py
• shellcheck -S error -f gcc scripts/build_with_cache.sh scripts/tests/test_build_cache.sh
• bash -n scripts/build_with_cache.sh scripts/tests/test_build_cache.sh
• python3 scripts/tests/check_text_integrity.py
• python3 scripts/tests/check_commit_messages.py
• git diff --check