Initial skeleton: ZMK module structure + Javelin submodule + stub shims

Phase 1 scaffold for porting Javelin steno engine as a ZMK module.
All platform shims are stubs — compiles but no steno processing yet.

src/platform/zmk_clock_shim.cc

@@ -0,0 +1,19 @@
+#include "clock.h"
+
+// TODO Phase 2: Replace with Zephyr kernel time functions.
+// #include <zephyr/kernel.h>
+// uint32_t Clock::GetMilliseconds() { return k_uptime_get_32(); }
+// uint32_t Clock::GetMicroseconds() { return k_cyc_to_us_floor32(k_cycle_get_32()); }
+// void Clock::Sleep(uint32_t ms) { k_msleep(ms); }
+
+uint32_t Clock::GetMilliseconds() {
+  return 0;
+}
+
+uint32_t Clock::GetMicroseconds() {
+  return 0;
+}
+
+void Clock::Sleep(uint32_t milliseconds) {
+  (void)milliseconds;
+}