Prepare HotWire 2.0.0a1 alpha release

[Monotoba]

Scope

• Correct steady-state wire resistance/heat-balance inputs and add consistency tests.
• Replace unsupported safety and production claims with explicit model assumptions and limitations.
• Update source installation and alpha release docs; remove legacy automatic PyPI/native-installer release workflows.
• Build and smoke-test a local sdist/wheel.

Local checks

• 65 tests passed with QT_QPA_PLATFORM=offscreen.
• Black and mypy passed.
• sdist and wheel built; twine check passed; clean Linux Python 3.12 wheel install, CLI and headless GUI smoke passed.

Remaining before release

• CI matrix on this PR.
• Review Windows installation and the alpha release notes/tag.
• No measured wire-temperature or foam-emissions validation has been performed. This is experimental alpha software.

This PR is a release candidate for review, not authorization to merge, publish on PyPI, or create a GitHub Release.