v1.4.0-rp2040 — persistent games (resume across power-off)

The board now remembers your game across power-offs. Play a few moves, switch the board off, come back hours or days later, power on, and it silently resumes exactly where you left off — no resume menu, no "set up the pieces" prompt.

Added

• Persistent games. After every move the board saves the full position, whose turn it is, castling rights, en-passant state, halfmove clock, and the mode/difficulty to a reserved 64 KB region of internal flash (mbed TDBStore over FlashIAP).
• Silent resume on boot. If a game is saved, the board adopts it directly — in AI mode it reconnects to WiFi automatically.
• The saved game stays active until you reset by placing all 32 pieces back on their starting squares (~1.5 s), which clears it and returns to the menu.
• Works in both Human-vs-Human and Human-vs-AI.

Notes

• The save survives a power cycle. Re-flashing the firmware (a new .uf2) may erase it — that only happens when you update, not during normal play.
• Leave the physical pieces where they are when you power off; on resume the board expects them to match the saved position.

Verified on hardware: a full power-cycle in the middle of an AI game resumed to the correct position and turn and reconnected to WiFi.

Includes everything from v1.3.0 (runtime difficulty, turn indicator, AI castling, valid-FEN fix, clean serial). Full details in CHANGELOG.md.