fix(build): name a Python that cannot start, and never reuse an old record

[MotimotiNotch]

Closes #5

What changes

• A Python that cannot start is named. When the interpreter in "python" cannot be started (ENOENT, or the Windows Store stub that exits with 9009 / prints "Python was not found"), the build stops with:

  could not start Python to trace scripts/weekly_summary.py ("python": "..." in the config).
  Install Python 3, or set "python" in loopfinder/config.json to the interpreter (for example "py" or a full path).
  

  instead of trace did not finish: ... (exit null).
• An old record is never used in place of this run's. Each trace's previous record is removed before the script runs. Before, if the script failed to start, a record left by an earlier build counted as this run's result, and the demo's declared loops were then reported as "declared but not found".

Not done: skipping .py traces and building from the rest. The declared loops that depend on those traces would be reported as "declared but not found" again, which looks like a broken flow rather than a missing interpreter.

Checked

• New test in test/core.test.js: with a Python that cannot start and a record left from an earlier build, the build throws a message naming the setting, and the old record is gone. It fails on main and passes here; all 27 tests pass.
• The steps in Building the demo without Python stops with an unclear error / Python が無い環境でデモをビルドすると分かりにくいエラーで止まる #5 with the CLI on Windows, with and without a record from an earlier build: both stop with the message above.
• Not reproduced: the Windows Store stub (exit 9009), as this machine has Python installed. Covered by the condition only.

The VS Code extension copies the CLI when it is packaged, so its users get this with the next extension release.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CS2kGo81duzNHgecBAWHe4