[codex] Add historical cost tracking

[LordMike]

Fixes #28

Summary

Adds opt-in historical cost tracking backed by one private Home Assistant Store per WattPlan config entry (wattplan.history.<entry_id>). The store keeps explicit UTC slot starts, one shared set of raw slot facts, self-consumption simulation outputs, and persisted self-consumption battery SoC state with fixed 60-day local-day retention.

Scenario semantics

• actual: measured grid import cost minus measured grid export value.
• no_battery: recomputed from retained usage/PV facts without duplicate raw storage.
• self_consumption: PV serves usage first, PV surplus charges configured batteries in order, batteries discharge before grid import, with no grid charging, no price awareness, and no preserve behavior.

Design notes

• Uses HA Store rather than recorder, SQLite, or backfill because V1 only needs a compact WattPlan-owned retention window and must avoid rewriting historical simulated results from later config changes.
• Saves are delayed/coalesced and flushed on unload or Home Assistant stop.
• Skipped slots, meter resets, missing/non-numeric meters, and missing prices are flagged as gaps/missing slots instead of spreading meter deltas across varying prices.
• Historical tracking is independent of planner execution and aligned to the configured slot size.
• EV availability/presence is intentionally deferred to Support EV battery availability and controllability #32.

Entity changes

Adds daily historical cost/savings sensors enabled by default when historical tracking is enabled, with monthly variants disabled by default. Removes the user-facing projected savings HA entities from setup/specs/docs/tests; existing HA registry entries are left for user cleanup.

Tests run

• WATTPLAN_TEST_VENV=/tmp/wattplan-venv-shared ./scripts/run_tests.sh tests/integration/test_config_flow.py tests/integration/test_integration_runtime.py tests/optimizer
• WATTPLAN_TEST_VENV=/tmp/wattplan-venv-shared ./scripts/run_tests.sh

[LordMike]

Review findings from PR #33:

• P1 custom_components/wattplan/historical_cost/tracker.py:213 / :227 / :150: first seed can create a bogus partial slot. If tracking is enabled at 12:30, _async_seed() reads meters at 12:30 but marks last_processed_slot as 11:00. At 13:05 it records slot 12:00 using the 12:30-13:05 meter delta. That contaminates the first day/month after enable, restart with missing cursors, or meter config changes.

• P2 custom_components/wattplan/historical_cost/tracker.py:328: export price “not used” no longer behaves as zero when an export meter is configured. _async_export_price() returns None, then the slot gets FLAG_MISSING_EXPORT_PRICE, making all scenario costs unavailable. This contradicts README.md:37 and planner behavior where missing export price means exported power value is zero.

• P2 custom_components/wattplan/historical_cost/simulations.py:98: self-consumption discharge capacity is reduced by efficiency twice. max_discharge_kwh is already delivered kWh per slot per docs/optimizer-api.md, and SoC drop already applies output / efficiency, so max_output = battery.max_discharge_kwh * efficiency underestimates discharge and biases the self-consumption baseline.

• P2 custom_components/wattplan/historical_cost/tracker.py:152: skipped intervals are under-counted. If Home Assistant misses several slots, the tracker appends one gap at the latest completed slot and seeds past the rest, so missing_slots reports 1 instead of the number of missed slot starts.

I could not run local tests from this environment because command execution could not start a shell, but GitHub CI for head 112a4a4 reports success.