feat: Two-player Mastermind game

[ap0ught]

Adds two-player Mastermind code-breaking game for linked CYDs with synchronized game transitions and round persistence.

[Copilot]

Pull request overview

Adds a synchronized two-player Mastermind game mode alongside the existing Greek Slide puzzle, including new protocol message types, shared epoch-based game selection, and UI/rendering paths for Mastermind on linked CYD devices.

Changes:

• Introduces Mastermind core state machine + validation/transition helpers and corresponding CI test coverage.
• Extends the ESP-NOW protocol with Mastermind state/full-state/ack/request packets and integrates Mastermind UI + game flow into src/main.cpp.
• Updates home screen UX and README to reflect a multi-game collection and the security/recovery model.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/mastermind_logic_test.cpp	Adds deterministic assertion-based coverage for Mastermind logic, transitions, and game selection clock helpers.
src/main.cpp	Implements Mastermind UI, input handling, state synchronization, epoch-based active-game arbitration, and protocol handling.
README.md	Updates project positioning to “two-player games”, documents Mastermind rules and security/recovery expectations.
include/protocol.h	Bumps protocol version and adds Mastermind wire packet definitions + size assertions.
include/mastermind_logic.h	Adds Mastermind state model, validation, transition rules, digesting, and reconciliation helpers.
include/game_selection.h	Adds shared “active game clock” helpers to arbitrate between concurrent game epochs.
.github/workflows/build.yml	Extends CI to compile/run Mastermind logic tests in addition to puzzle tests.

Suppressed comments (1)

src/main.cpp:1206

• protocolMutex guards other protocol send paths, but the Mastermind state-request send is unguarded. If the mutex is required to serialize ESP-NOW access with the receive callback and other sends, this path should take it too for consistency.

void sendMastermindStateRequest() {
    uint32_t gameId = 0;
    uint32_t revision = 0;
    uint32_t digest = 0;
    portENTER_CRITICAL(&gameMux);
    if (mastermindStateReady) {
        gameId = mastermindState.gameId;
        revision = mastermindState.revision;
        digest = mastermindStateDigest(mastermindState);
    }
    portEXIT_CRITICAL(&gameMux);
    MastermindStateRequestPacket packet{
        makeHeader(MessageType::MastermindRequestState), gameId, revision,
        digest};
    esp_now_send(expectedPeerAddress, reinterpret_cast<uint8_t*>(&packet),
                 sizeof(packet));

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