fix(controller): floor proactive_export reserve at minimum_target_soc on SOC=0 (#895)

[jackmcintyre]

Summary

set_proactive_export used reserve = max(4.0, current_soc - 5.0). When SOC=0 (entity unavailable, startup, or genuinely empty), this yielded reserve=4.0, draining the battery to 4% — well below the user's configured minimum_target_soc. A user who set minimum_target_soc=30 to protect reserve would still see the battery drained to 4% during proactive export.

Live evidence: select.localshift_battery_mode = self_consumption (latent for this install), but the manual selection path has no SOC validation.

Changes

integration/controller.py:set_proactive_export:

• When SOC <= 0: floor reserve at minimum_target_soc (via _get_minimum_target_soc from fix(controller): read minimum_target_soc from config option, not data entity (#894) #894) and emit a WARNING so the SOC=0 case is diagnosable.
• Otherwise: dynamic reserve = max(minimum_target_soc, SOC - 5). The configured minimum always holds; the old hardcoded 4.0 floor is gone.

Dependencies

Cherry-picks #894 (read minimum_target_soc from config option). That fix is required for this one to floor at the correct (configured) value rather than the old default. If #894 lands first, this PR's cherry-pick commit should be dropped on merge.

Validation

• TDD: 2 new failing tests (test_set_proactive_export_soc_zero_floors_at_minimum_target, test_set_proactive_export_soc_zero_dry_run_floors_at_minimum), then fix.
• Updated test_set_proactive_export_minimum_reserve which encoded the old 4.0 floor — now expects DEFAULT_MINIMUM_TARGET_SOC (20).
• Full suite: 3219 passed, 1 xfailed. Coverage: 95%.
• uv run ruff check clean.

Test plan

• Unit tests pass
• Coverage ≥95%
• Deploy + verify reserve never drops below configured minimum during proactive export