docs: Venus A production-measured efficiency curve + DB analysis methodology

[bvweerd]

Summary

• Adds a production-measured efficiency curve for the Marstek Venus A, derived from HA recorder data on two units over several weeks (Sept 2026)
• Adds docs/efficiency-analysis-from-db.md: step-by-step methodology for repeating the analysis in future

How the curve was derived

Two metrics from the HA recorder database:

1. Dispatch fidelity (hourly SQL on statistics table): actual AC energy / commanded energy

   • Discharge: ~93.7 % (flat across all SoC bands)
   • Charge: ~99 % up to 80 % SoC, ~89 % near full (CV taper)

2. SoC-based efficiency correction (.storage/battery_controller_*_eff files): both units showed efficiency_factor > 1.0 — battery beats the community curve — with correction capped at 1.0492 and applied: False

Community curve × 1.0492:

Power	Community one-way	Production one-way	Community RTE	Production RTE
100 W	0.764	0.802	58 %	64 %
500 W	0.909	0.954	83 %	91 %
800 W	0.908	0.953	82 %	91 %
1200 W	0.892	0.936	80 %	88 %

Test plan

• No Python code changed — no tests to run
• Verify efficiency-curves.md renders correctly in GitHub preview
• Verify efficiency-analysis-from-db.md SQL query runs against a local HA DB copy