fix(slots): handle cross-midnight demand windows (#896)

[jackmcintyre]

Summary

The DW membership test dw_start_time <= slot_time < dw_end_time is false for every slot of the day when end <= start (e.g. 22:00 → 06:00 overnight peak). The feature silently disabled itself — no slot flagged, no DW entry detected, no pre-charge — with no error or warning.

_parse_time_option (slots.py:473) does no cross-midnight handling, and there's no validation in config_flow. The default 15:00–21:00 doesn't cross midnight, so this is latent for most users but bites anyone on an overnight-peak tariff.

Changes

engine/slots.py:_process_single_slot detects the cross-midnight case (end <= start) and applies the wrap:

if dw_end_time <= dw_start_time:
    in_demand_window = slot_time >= dw_start_time or slot_time < dw_end_time
else:
    in_demand_window = dw_start_time <= slot_time < dw_end_time

Validation

• TDD: 5 new tests in TestDemandWindowCrossMidnight — slots at 23:00 (in), 02:00 (in), 12:00 (out), 22:00 (entry boundary), 06:00 (exit, exclusive). 3 failed red (23:00, 02:00, 22:00 all wrongly False), all green after fix.
• Full suite: 3219 passed, 1 xfailed. Coverage: 95%.
• uv run ruff check clean on slots.py (pre-existing import-sort warnings in test file are unrelated).

Out of scope

Config-flow validation (warning the user when end <= start) is intentionally not added — overnight windows are legitimate, so the fix is to handle them correctly rather than block them.