Doc-Guardian: document screen reader speed event/key additions from PR #25

[Copilot]

PR #25 adds a new screen reader speed notification bridge and a corresponding persistence key, but documentation did not reflect those API/architecture changes. This update closes that documentation gap in product and technical docs.

• Product documentation (PRODUCT.md)

  • Added accessibility coverage for OnScreenReaderSpeedChanged (uint8 speed payload).
  • Updated JSON-RPC event capabilities to include forwarding of this new event to clients.

• Architecture documentation (ARCHITECTURE.md)

  • Added screenReaderSpeed under accessibility settings as a persisted key.
  • Documented notification bridging path for OnScreenReaderSpeedChanged(...) via Exchange::JUserSettings::Event::*.
  • Marked end-to-end producer/consumer verification as [Verification Needed] where implementation context is outside the current diff.

void OnScreenReaderSpeedChanged(const uint8_t speed) override
{
    LOGINFO("ScreenReaderSpeedChanged: %d\n", speed);
    Exchange::JUserSettings::Event::OnScreenReaderSpeedChanged(_parent, speed);
}