P0: Wire http_api.py (FastAPI) as the main entrypoint for python -m heidi_engine.http

[heidi-dang]

FastAPI module loads but is not the main entrypoint yet; runtime is still legacy http.py.

Acceptance:

• python -m heidi_engine.http launches FastAPI by default
• Legacy remains accessible only via explicit flag (e.g. --impl=legacy) or separate module
• If HTTP extras missing, print actionable hint: pip install -e '.[http]'
• Default bind remains localhost-only (127.0.0.1)
• CI includes a minimal smoke/import test for the entrypoint.