docs: update README with all 9 completed plans and PR references

[basmeerman]

Summary

README updates

• Add Plan 06 (Diagnostic Telemetry) features section — ring buffer, LittleFS persistence, WebSocket stream, test replay, MQTT profile control (PR feat: diagnostic telemetry system (Plan 06) #84)
• Add Plan 07 (Web UI Modernization) features — offline-first, WebSocket data, dark mode, power flow diagram, node overview, diagnostic viewer (PR feat: Web UI modernization — all 8 increments (#51-#58) #85)
• Add Plan 09 (Power Input Methods) features — API staleness detection, HomeWizard P1 energy/manual IP, metering diagnostic counters, comprehensive docs (PR feat: power input methods — docs, staleness, HW energy, manual IP, diagnostics (#71-#75) #86)
• Add per-phase energy and metering diagnostic counters to MQTT section
• Update Testing & Quality metrics: 43 suites, 870+ scenarios
• Update Roadmap: all 9 plans marked Done with PR links for traceability

Settings reference appendix (configuration.md)

• Comprehensive reference of all SmartEVSE settings across all access channels
• Access matrix: LCD, Web UI, REST API, MQTT — read/write per setting
• 14 safety-relevant settings flagged with ⚠ warnings and explanations
• NVS persistence info (survives reboot or not)
• Read-only status values section
• Safety summary with recommendation to use LCD for initial setup

Test plan

• Documentation-only changes — no code modified
• All PR links verified to exist
• Settings reference cross-checked against source code (main.h, mqtt_parser.c, http_api.c, esp32.cpp)

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