keel positions: attributed DCA tranches show a stop distance measured against the stop=0 sentinel #973

Found while reviewing PR #972 (round 1, held H3). Pre-existing.

agent._open_tranche copies Dca.detect's stop=0 sentinel onto every attributed live DCA tranche (live ledger: tranches 4-6 store initial_stop "0"). keel/commands/positions.py (module docstring ~line 25, field docstring ~line 120) says initial_stop is None for a DCA leg, and computes stop_distance = mark - initial_stop (~line 402). For these tranches that makes the distance equal to the mark, which is the "distance measured against a substituted zero" the module docstring warns against. doctor.py ~1813 repeats the "DCA initial_stop absent" claim, although its > 0 guard makes it harmless.

Fix: read the stop through status.recorded_stop (added in #972: zero or below counts as no stop) in positions.py, and correct the docstrings in positions.py and doctor.py.