docs(plans): add direction plans 014-015 with execution index

[G4brym]

Summary

Third /improve next (direction audit) run, planned against e6d6454. Adds two selected direction plans and reconciles the execution index.

• Plan 014 - Persist entity state (P1, M): emitState is currently broadcast-only (deviceSession.ts:574) - nothing is stored, so sensor readings are invisible unless a watcher is open. Adds a history table with retention + write coalescing (mirroring the device_logs pattern), a last-known-value table that survives retention, watcher replay of last-known state on attach, GET .../state + .../state/history endpoints, and a dashboard State tab with echarts history charts. Best landed before plan 001 (HA integration) executes so HA gets state restore on reconnect from day one.
• Plan 015 - Flash devices from the dashboard (P2, M): dashboard-only WebSerial flashing for ESP32 via esptool-js (the server already streams fully patched, checksum-corrected images) plus one-click patched UF2 download for Pico. Feature-detects the secure-context requirement and falls back to download + CLI instructions until plan 008 (TLS) ships.
• Index: rows for 014/015, dependency notes (014↔001/007/012, 015↔008/010), and the round's not-selected findings recorded (webhook/ntfy alerting, CodeMirror script editor, Pico OTA deferred until 010 lands) so future audits don't re-litigate them.

Plans only - no source code changes, hence no changeset (same as #215-#217).

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