impl/system/linux: ensure num_cores() returns a non-zero value.
system: added time_monotonic(); updated test/system/system.
gfx/display: moved input_buffer private func implementations to .cpp.
input_buffer: added input state persistence.
input: added input state interface.
test/gfx/display: simplified input handling.
test/common/helpers: renamed core_init() to memory_init().
app: new module; stubbed App class.
test/app/app: new test.