Implement Phase 2: real platform shims + behavior driver + steno key defines

- zmk_key_shim: Key::Press/Release → raise_zmk_keycode_state_changed()
  Maps Javelin KeyCode to ZMK HID events (keyboard + consumer pages)
- zmk_clock_shim: Clock:: → Zephyr k_uptime_get/k_cyc_to_us/k_msleep
- zmk_flash_shim: Flash::EraseBlock/WriteBlock → Zephyr flash API
- behavior_javelin_steno.c: proper DT_DRV_COMPAT, DT_INST_FOREACH_STATUS_OKAY
- engine_init.cc: processor pipeline wiring (AllUp → JeffModifiers → Engine)
- javelin_steno.dtsi: default &javsteno behavior instance
- dt-bindings/zmk/javelin_steno.h: STENO_S1..STENO_ZR key defines for keymaps
- Exclude thread.cc (pthreads), key.cc, clock.cc (replaced by shims)
- Fix CMake: combine COMPILE_FLAGS + COMPILE_DEFINITIONS in single call

Engine init deferred until dictionary loaded (Phase 3).

dts/behaviors/javelin_steno.dtsi

@@ -0,0 +1,8 @@
+/ {
+    behaviors {
+        /omit-if-no-ref/ javsteno: javelin_steno {
+            compatible = "zmk,behavior-javelin-steno";
+            #binding-cells = <1>;
+        };
+    };
+};