Add synchronized two-player Battleship

[ap0ught]

Feature

Add synchronized two-player Battleship for the paired ESP32-2432S028 CYDs.

Gameplay

• Each player privately places a fleet on their own grid.
• After both fleets are committed, players alternate firing at coordinates on the opponent’s grid.
• The defending device validates each shot against its private fleet and returns miss, hit, or sunk.
• A player wins after every opposing ship is confirmed sunk.
• Do not declare victory until the complete fleet state and final shot are validated.

Board modes

• Classic 10×10.
• Compact 8×8.
• Quick 6×6.
• Fleet composition must scale appropriately for each board size.

Placement rules

• Ships are straight, contiguous, in bounds, and non-overlapping.
• Support touch placement, rotation, movement, and a final ready/commit action.
• Keep uncommitted placement editable.
• Do not reveal ship locations through the opponent’s normal UI or protocol responses.

Turn and shot rules

• Enforce authenticated turn ownership.
• Reject repeated, out-of-bounds, stale, and out-of-turn shots.
• Track misses, hits, sunk ships, remaining fleet, and match completion deterministically.
• Optional future modes such as Salvo or Sonar must not alter classic rules by default.

Synchronization and privacy

• Use fixed encrypted ESP-NOW peers, authenticated sender identity, active-game epochs, replay rejection, ACK/retry, and deterministic reconciliation.
• Prefer each board remaining authoritative for its own private fleet rather than synchronizing plaintext fleet positions to the opponent.
• Design restart recovery without revealing hidden placement.
• Document the trusted-device threat model: modified firmware or physical extraction from a paired board is outside ordinary UI secrecy guarantees.
• Define deterministic handling for simultaneous events, disconnects, and restart during placement or combat.

Navigation

• Both boards provide an EXIT control during placement, combat, and results.
• Either player may return both devices to the shared Home selector.
• EXIT dominates shots and placement commits, retries after packet/ACK loss, and cannot be undone by stale packets.

Acceptance criteria

• Fleet placement validation covers bounds, orientation, overlap, and fleet composition.
• Hidden fleets are not exposed to the opponent’s standard UI or routine shot messages.
• Turn ownership, shots, hit/miss/sunk responses, and victory are deterministic.
• All three board sizes remain readable and touchable on the 320×240 display.
• Restart and reconciliation preserve privacy or fail closed.
• EXIT works from either board in every phase.
• Pure rules pass strict host compilation with -std=c++17 -Wall -Wextra -Werror.
• Encrypted PlatformIO firmware builds successfully.
• Both CYDs are flashed and physically verified through placement, combat, sinking, victory, restart, and bilateral EXIT.