npm wrapper: venv bootstrap hard-codes `python3`, fails on Windows installs without a python3 command

[Scottcjn]

Defect

The npm wrapper always creates its venv with python3, including on Windows, where a standard Python install usually provides python.exe / py.exe but no python3 command.

Where

• bin/beacon.js:25-30 — pythonBin() already handles win32 (Scripts/python.exe)
• bin/beacon.js:48-54 — ensureVenv() runs run('python3', ['-m', 'venv', VENV_DIR]) unconditionally
• bin/beacon.js:42 — run() throws on spawn error, so a missing python3 aborts the first run (main @ d975e13)

Repro

1. On Windows with Python from python.org (no python3 alias), with ~/.beacon/npm/venv absent
2. Run beacon <anything>
3. spawnSync('python3', ...) fails with ENOENT before the venv exists

Expected vs actual

• Expected: bootstrap succeeds, since the wrapper already supports the Windows venv layout
• Actual: bootstrap fails on Windows systems without a python3 command

Suggested fix

Choose the bootstrap interpreter per platform: honor PYTHON if set, then try py -3 / python on win32 and python3 / python elsewhere. Add a unit test for the selection.

No open fix PR for this was found at filing time (checked gh pr list for fable/*-2026-09-21 branches).

Reported by @fsalmon1991 via email (GitHub App 403 fallback) for rustchain-bounties#254; filed on their behalf by Sophia (Elyan Labs).

AI-assisted: the reporter disclosed AI assistance; this issue was verified against current main and drafted with AI assistance (Claude) before filing.

[jmatson36097-blip]

🛠️ Solution Package & Pull Request

Hi maintainers! I've opened PR #946 addressing this issue:

• Pull Request: #946
• Changes: Updated bin/beacon.js to resolve bootstrap Python interpreter across Windows (py -3 / python) and Unix (python3 / python), while honoring any explicit PYTHON env variable.
• Verification & Tests: Added unit tests in tests/test_npm_wrapper.py covering interpreter selection. Tests passed cleanly.