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).

include/zmk_javelin_steno/zmk_platform_shim.h

@@ -9,4 +9,12 @@ void zmk_javelin_steno_process_key(int steno_key_index, bool is_press);
 
 #ifdef __cplusplus
 }
+
+class StenoDictionary;
+struct StenoOrthography;
+
+// Complete engine initialization with loaded dictionary.
+// Called from dictionary loading code once flash data is validated.
+void zmk_javelin_steno_init_engine(StenoDictionary &dictionary,
+                                   const StenoOrthography &orthography);
 #endif