[S1] Backend package not installable — closed_tokens.py imports ucns_v04 from repo root

[erinepshovel-code]

backend/src/edcmbone/closed_tokens.py contains:

from ucns_v04 import (UCNSObject, AnchorPayload, unit_obj, multiply,)

ucns_v04.py lives at the repo root, not inside backend/src/edcmbone/. After pip install -e ./backend, importing any module that touches closed_tokens raises:

ModuleNotFoundError: No module named 'ucns_v04'

The "87 tests passing" figure in CLAUDE.md holds only because tests are run from the repo root with ucns_v04.py reachable via cwd. The Backend package is not independently installable.

Options (ascending disruption):

1. Vendor ucns_v04.py into backend/src/edcmbone/ and switch to from .ucns_v04 import ... — most self-contained.
2. Declare ucns_v04 as a proper dependency in backend/pyproject.toml, installable from a separate package.
3. Document in CLAUDE.md that the Backend package requires repo-root cwd — accepts the limitation but at least makes it explicit.