Harden one-command startup

[temiroff]

What changed

• isolate Windows startup dependencies in the repository .venv, matching macOS/Linux
• add bootstrap-only launcher mode and a Windows clean-bootstrap CI job
• keep optional CUDA installation owned by the CUDA extension package
• repair stale editable installs by checking that blacknode is importable
• document clone-to-launch as the primary onboarding path
• add launcher contract regression tests

Why

Blacknode already supported clone-and-launch onboarding, but Windows installed into the active global Python environment and the public walkthrough still required manual pip and npm commands. This change makes the one-command path isolated, testable, and accurately documented.

Validation

• first isolated Windows bootstrap: 56.2 seconds
• warm bootstrap: 2.6 seconds
• backend and editor ready: 13.9 seconds, both HTTP 200
• blacknode demo: passed (Hello World)
• python -m pytest -q: 591 passed, 16 skipped, 45 subtests passed
• npm run build: passed
• PowerShell and shell syntax checks: passed
• CI workflow YAML parse: passed